Include works?
Now that I think about it, can't you just add "&forumid=x,y,z" into the query like you do for the rss?
Or, now that I think again.... look at the vb.org page where it says "Click here to view latest topics" -
https://vborg.vbsupport.ru/search.ph...aveprefs=false - so they use forumid and then childforums=1 to include the childforums.
Why didn't I think of that before?

(Thanks for getting me to rethink this, Videx!)