So, new "whatnowproc" is $HOME/bin/senddraft, which contains "send -draft" and I think this works for me.
So I have:
(defun mcr-forward-to-fastmail () "Forwards current email to FastMail Account,
no outbox"
(interactive)
(let ((msglist (mh-interactive-range "")))
(mh-exec-cmd "dist" "-draftfolder" "+drafts" "-draftmessage" "new"
"-whatnowproc" "senddraft" "-form" "fmcomps"
mh-current-folder msglist)))
% cat $HOME/bin/senddraft
#!/bin/sh
send -draft "$@"
