Hi,
in Locus itself is not any possibility to mark more then one cache at once as "computed". If you export such cache to GPX file, you should see that there is also not any special GPX tag. Unfortunately, Groundspeak isn't able to create some improved version of <groundspeak:> tags with support for some other features. Anyway you may notice, that together with cache itself, there is also exported one waypoint with name "Final" and code "FI". If you create also such waypoint where code start with "FI" (even a "FIxxxxx" should work) ... it should after import move cache and mark it as computed. Anyway I'm not sure if this is simple solution for you