Are there any fora for conversing in Classical Chinese?
新文言
辞赋论坛
Hummm, I cannot find a forum where people converse In 文言文, but the two fora above are mainly for writing In 文言文.
I must say, 文言文 doesn't quite fit into modern environment. The main reason is words for modern things and ideas
are formed in such a different way from the morphology of 文言文 that it looks awkward for a pure 文言文 to contain modern words like 電話/牙刷/激光照排打印機.

Whereas it is possible to elaborate your speech or writing with 文言文, including using 文言句式/文言詞彙/典故/文風 and In many cases it reads really like so called 文言文 （of course it isn't）.

——————EDIT——————

I think the essence of good writing and speech is concision and accuracy. The more words at your command, the nearer you are from that goal. In daily life and on the Internet, people are inclined to use basic and simple words. If you are not a beginner, you will feel this a hindrance to your improvement. It is, even for native speakers. It might be the reason why you are looking for fora discussing Classical Chinese. But it is not necessaryԅ(¯ㅂ¯ԅ). My advice is simple but proved by many: read books writing in formal and literary language and （my personal practice） buy a hefty dictionary, often leaf through it, take down notes of good words you meet.
|･ω･｀)