Guess I was wrong.

There is a fairly-new "Set Playlist Order" feature, but it doesn't write to the track number field that I can tell, sorry.

Perhaps I was thinking of a Jemplode feature? If not, seems like it would be do-able.

Another option is to download the tracks from Jemplode using a POS:2 field and then do a filenames-to-tags pass on it with MP3 Tag Studio.