It's a local command:
"C:\Program Files\WinMerge\WinMergeU.exe" /x /u "!^!" "!"
Note that I'm able to use a local text editor to edit a selected remote file, and on saving the remote file from the local editor, it is then copied to the remote. This works perfectly (and is extremely useful, by the way). Is there a way to have this upload happen with custom (local) commands?