2. why would it be completely useless? You just have to create a rule in TB to move all emails with the subject tag to your folder of choice.
that's a cool and simple idea and I really shame myself for not having thought about it

, especially considering I've been "playing" with filters in Thunderbird during the last few days

I deactivated AIS spam filter a while ago and didn't look back really. Okay Vlk, thanks for posting that obvious tip

This said, I still have a problem with that, once a mail has been marked as spam by Avast, and it was a mistake, there's no way to undo that, i.e. undo what's been added to the subject line of the mail. When that happened here a while ago, I found that the only way out was to re-download the mail (this now tagged subject line doesn't synchronize itself in IMAP fortunately). Is there another way to un-tag if needed, directly in the client?
edit: filtering with the Avast subject tag doesn't work :'( ... the avast spam tag is seen in the subject line when opening a mail marked as such, it does appear in the message source,
but it doesn't appear in the mail entry subject line in the list of mails, explaining why the tag doesn't synch in IMAP probably (in the Gmail interface, the message source doesn't synch either obviously). The Avast spam tag must be added after the mail has been received by the client...and anyway, even running the filter manually after that doesn't work.
edit: just for info, moving a mail marked as spam by avast to a local folder, then back to the imap folder where it came from, then rebuilding the index if that imap folder let the avast spam tag appear in the subject line in the mail list too, and then the filter works, but in these conditions

edit: the avast spam tag seems to be added only at local level, just like the mail shield inserted note that never synches either in IMAP.