Better to make another file (say) providers-overlays.xml and place in that directory. This will be read as well as the standard providers.xml from Locus tweaks and avoids any problems of messing up the original file.
Edit: Attached is my providers-overlays.xml file. Download it and drop it into the locus\mapsonline\custom folder. It includes my modified versions of the various methods suggested so far. Remember to set the "Valid for" to 1 hour for the weather and 0 for the Traffic, as described previously. There are a number of overlay versions (Traffic, Clouds, Icons, Clouds and Icons, Icons and Clouds, of which the last one can give lots of missing tiles - though setting "Valid for" to 0 for this one vastly improves this) and a standalone map, which appears under Google in the map dialogue.
<?xml version="1.0" encoding="UTF-8"?>
<providers>
<provider id="11701" type="0" visible="true" background="-1">
<name>Overlays</name>
<mode>Traffic</mode>
<area></area>
<url><![CDATA[http://mts{s}.google.com/mapslt?src=app&lyrs=traffic&x={x}&y={y}&z={z}&style=15&w=256&h=256]]></url>
<serverPart>0;1;2;3</serverPart>
<zoomPart>{z}-8</zoomPart>
<zoomMin>12</zoomMin>
<zoomMax>23</zoomMax>
<tileSize>256</tileSize>
<attribution><![CDATA[(c)2013 Google]]></attribution>
<extraHeader><![CDATA[User-Agent#Mozilla/5.0 (Windows NT 6.1; WOW64; rv:7.0.1) Gecko/20100101 Firefox/7.0.1]]></extraHeader>
<extraHeader><![CDATA[Referer#http://maps.google.com/]]></extraHeader>
</provider>
<provider id="11704" type="0" visible="true" background="-1">
<name>Overlays</name>
<mode>Clouds</mode>
<area></area>
<url><![CDATA[http://mts{s}.google.com/mapslt?src=app&lyrs=weather_0cloud&src=app&x={x}&y={y}&z={z}&s=Galileo]]></url>
<serverPart>0;1;2;3</serverPart>
<zoomPart>{z}-8</zoomPart>
<zoomMin>8</zoomMin>
<zoomMax>14</zoomMax>
<tileSize>256</tileSize>
<attribution><![CDATA[(c)2013 Google]]></attribution>
<extraHeader><![CDATA[User-Agent#Mozilla/5.0 (Windows NT 6.1; WOW64; rv:7.0.1) Gecko/20100101 Firefox/7.0.1]]></extraHeader>
<extraHeader><![CDATA[Referer#http://maps.google.com/]]></extraHeader>
</provider>
<provider id="11705" type="0" visible="true" background="-1">
<name>Overlays</name>
<mode>Weather Icons</mode>
<area></area>
<url><![CDATA[http://mts{s}.google.com/mapslt?src=app&x={x}&y={y}&z={z}&s=Galileo&lyrs=weather_c_mph|invert:1&s=Galileo]]></url>
<serverPart>0;1;2;3</serverPart>
<zoomPart>{z}-8</zoomPart>
<zoomMin>8</zoomMin>
<zoomMax>26</zoomMax>
<tileSize>256</tileSize>
<attribution><![CDATA[(c)2013 Google]]></attribution>
<extraHeader><![CDATA[User-Agent#Mozilla/5.0 (Windows NT 6.1; WOW64; rv:7.0.1) Gecko/20100101 Firefox/7.0.1]]></extraHeader>
<extraHeader><![CDATA[Referer#http://maps.google.com/]]></extraHeader>
</provider>
<provider id="11710" type="0" visible="true" background="-1">
<name>Overlays</name>
<mode>Clouds and Icons</mode>
<url><![CDATA[http://mts{s}.google.com/mapslt?src=app&x={x}&y={y}&z={z}&s=Galileo&lyrs=weather_0cloud,weather_c_mph|invert:1&s=Galileo]]></url>
<serverPart>0;1;2;3</serverPart>
<zoomPart>{z}-8</zoomPart>
<zoomMin>8</zoomMin>
<zoomMax>26</zoomMax>
<tileSize>256</tileSize>
<attribution><![CDATA[(c)2013 Google]]></attribution>
<extraHeader><![CDATA[User-Agent#Mozilla/5.0 (Windows NT 6.1; WOW64; rv:7.0.1) Gecko/20100101 Firefox/7.0.1]]></extraHeader>
<extraHeader><![CDATA[Referer#http://maps.google.com/]]></extraHeader>
</provider>
<provider id="11711" type="0" visible="false" required="false" background="-1">
<name>Overlays</name>
<mode>Clouds base</mode>
<url><![CDATA[http://mts{s}.google.com/mapslt?src=app&x={x}&y={y}&z={z}&lyrs=weather_0cloud&s=Galileo]]></url>
<serverPart>0;1;2;3</serverPart>
<zoomPart>{z}-8</zoomPart>
<zoomMin>8</zoomMin>
<zoomMax>14</zoomMax>
<tileSize>256</tileSize>
<attribution><![CDATA[(c)2013 Google]]></attribution>
<extraHeader><![CDATA[User-Agent#Mozilla/5.0 (Windows NT 6.1; WOW64; rv:7.0.1) Gecko/20100101 Firefox/7.0.1]]></extraHeader>
<extraHeader><![CDATA[Referer#http://maps.google.com/]]></extraHeader>
</provider>
<provider id="11712" type="0" visible="true" background="11711">
<name>Overlays</name>
<mode>Icons and Clouds</mode>
<url><![CDATA[http://mts{s}.google.com/mapslt?src=app&x={x}&y={y}&z={z}&lyrs=weather_c_mph|invert:0&s=Galileo]]></url>
<serverPart>0;1;2;3</serverPart>
<zoomPart>{z}-8</zoomPart>
<zoomMin>8</zoomMin>
<zoomMax>26</zoomMax>
<tileSize>256</tileSize>
<attribution><![CDATA[(c)2013 Google]]></attribution>
<extraHeader><![CDATA[User-Agent#Mozilla/5.0 (Windows NT 6.1; WOW64; rv:7.0.1) Gecko/20100101 Firefox/7.0.1]]></extraHeader>
<extraHeader><![CDATA[Referer#http://maps.google.com/]]></extraHeader>
</provider>
<provider id="11713" type="0" visible="false" required="false" background="-1">
<name>Google</name>
<mode>Satellite and Clouds</mode>
<url><![CDATA[http://mts{s}.google.com/mapslt?src=app&x={x}&y={y}&z={z}&lyrs=s,h@1,weather_nolabels,weather_0cloud&s=Galileo]]></url>
<serverPart>0;1;2;3</serverPart>
<zoomPart>{z}-8</zoomPart>
<zoomMin>8</zoomMin>
<zoomMax>14</zoomMax>
<tileSize>256</tileSize>
<attribution><![CDATA[(c)2013 Google]]></attribution>
<extraHeader><![CDATA[User-Agent#Mozilla/5.0 (Windows NT 6.1; WOW64; rv:7.0.1) Gecko/20100101 Firefox/7.0.1]]></extraHeader>
<extraHeader><![CDATA[Referer#http://maps.google.com/]]></extraHeader>
</provider>
<provider id="11714" type="0" visible="true" background="11713">
<name>Google</name>
<mode>Weather Satellite</mode>
<url><![CDATA[http://mts{s}.google.com/mapslt?src=app&x={x}&y={y}&z={z}&lyrs=weather_c_mph|invert:0&s=Galileo]]></url>
<serverPart>0;1;2;3</serverPart>
<zoomPart>{z}-8</zoomPart>
<zoomMin>8</zoomMin>
<zoomMax>26</zoomMax>
<tileSize>256</tileSize>
<attribution><![CDATA[(c)2013 Google - Map data (c)2011 Tele Atlas]]></attribution>
<extraHeader><![CDATA[User-Agent#Mozilla/5.0 (Windows NT 6.1; WOW64; rv:7.0.1) Gecko/20100101 Firefox/7.0.1]]></extraHeader>
<extraHeader><![CDATA[Referer#http://maps.google.com/]]></extraHeader>
</provider>
</providers>