For projects it wont work because comments in projects are used for:
1) Commenting on the status of the task (along with hour information)
2) Providing additional information about a task / project
3) Asking questions for clarification
This is why the idea of threaded comments from dave is great, but I have to ask myself, if I have threaded forums, why do I need threaded comments? Isn't this code duplication? I think what would be easiest is if you could say that forum messages can have any object_id as a parent and then rewrite thread-chunk.tcl which takes care of that.