I have one suggestion, maybe its not very good, but no matter.
When replying to topics some forums require to provide topic title. Percentage of that kind of forums is very little, i have list of more than 300 forums but only 4 require topic title when replying.
My suggestion, on forum page implement some kind of checkbox ''use topic title when replying'', when checked when replying on topic from that forum tp will use title and on others will not, default is not checked.
Maybe its crappy suggestion, but its annoying making groups just for that kind of forums..

I think you can use the same title place as posting..
Just try it..

Yes, just use the subject field. It will be the title / subject of the reply.