Basic template+autoadd categories are easy enough with existing codebase. Queries would require an extension (SMW package), which is addable by anyone with access in a few minutes and allow fun things like editing the template with a form rather than raw wikitext. The part about user inactivity would need a custom extension which would be either relatively simple or really annoying depending on how the recent activity data can be retrieved from the reddit install. Actually, thinking again, if there's an API we can call for user's most recent activity it should be possible using the existing extension ExternalData and a clever template or two, no need for custom coding.

And yea, it seems like there should be a few people with the coding skills willing to help out, though from my exploration of the code it seems to be unchanging (unless the public version control is just not updated any more).