SFTP and FTP/SSL are completely different protocols.
FTP/SSL should be relatively easy now that VLC has a cross-platform generic SSL/TLS module.
SFTP would rely on a an SSH implementation - I'm not sure if there is any good GPL-compatible one, and I doubt we could additionnaly find a portable one.
It's an Internet draft at the moment. Doesn't seem to complicated.
I'll probably go the KDE way of implementing just SFTP (quite easy) and relying on the system's ssh for lower level communication. Unfortuntaly, it means it won't work on Windows (except with cygwin).
Hmm, I did try. Unfortunately, OpenSSH does not seem to support recent versions of the SSH protocol. This means that OpenSSH cannot handle filename characters set... which in turn mean we will only ever be able to open plain ASCII filenames on servers whose local encoding is not UTF-8.