Emule has a feature (Sparse NTFS files) that doesn’t allocate the full size of files on the hard drive until they are actually complete. Does Bitcomet have a similar feature? After adding a dozen huge torrents to a new 1tb driver, it is saying it is full after downloading only a few MBs.
The protocol used to download torrents was written by bittorrent inc, and making changes to it is a long complicated task and must not change the way existing clients and peers use the system, so I’d say no, you’ll need to have enough room on the drive before starting the torrent.