If I (legally) download a torrent file on Windows (running bootcamp) and select my mac harddrive as its download location on, say, uTorrent, will it work?
Yes if Windows can read the format of the Mac partition. Mac OS X uses HFS+, which Windows 7 cannot read without a 3rd party tool. According to this site you can use HFSexplorer which can be downloaded here.
Note: The program doesn't say anything about writing files, only reading, but it doesn't explicitly say that it cannot write. If this happens to be the case, someone let me know.