I have an idea. Is it possible to include 7z(LZMA) module, and 7z-ed the files during transmission?
User can choose some local files for upload, and
set 7z(LZMA) parameter in a dialog box, ex: compression level, encryption...., and then
WinSCP streaming the files to LZMA-->(WinSCP transmit module)-->remote.
Files are 7z-ed on the fly, user need not pre-7z before upload.
Similarly, when downloading, User can decide to un-7z on the fly. WinSCP pass the received data stream to LZMA-->(save un-7z data to local file system).
In some applications, the local disk space is limited, it’s impossible to keep both source and 7z-ed file.