We are encouraging posters to add more info to thread titles in the "Deals, Freebies" section - koland in particular is trying to squeeze as much as she can, and other posters seem to be following her lead. See here:
https://www.mobileread.com/forums/sho...33&postcount=1
Will this type of thread title be workable for you?
Adding prefixes can be challenging and time-consuming to apply & keep current, esp. for a forum in which threads for deals/offers appear & age off quickly.
One other thought would be to somewhat emulate the Kindle "Free Books" sticky thread - have 1 thread/month extensively labeled: "DRMFree Kindle SciFi Freebies", "DRM US-Only Kindle Romance" - and restart a new thread monthly.
Any way you go will be labor intensive.