Epoll on regular files
Not really. epoll only makes sense for file descriptors which would normally exhibit blocking behavior on read/write, like pipes and sockets. Normal file descriptors will always either return a result or end-of-file more or less immediately, so epoll wouldn't do anything useful for them.
I think, it will fail at epoll_ctl with EPERM:
EPERM The target file fd does not support epoll.if the file has no poll() interface.
The actual code is http://lxr.linux.no/#linux+v3.1/fs/eventpoll.c#L1373
1373 /* The target file descriptor must support poll */1374 error = -EPERM;1375 if (!tfile->f_op || !tfile->f_op->poll)1376 goto error_tgt_fput;1377