If you don't want to change the global defaults (in `/etc/xpra`), or your personal config (`~/.config/xpra`), you should be able to stick `ssh=ssh` in a `$HOST.xpra` session file and the launcher should honour it when you open the file / double-click on it.
(add `autoconnect=true` and the launcher won't be seen unless the connection fails)
The manpage implies you can't, but I tested and you can.
> and can't you pass the flag when you launch the gui?
I just tested and passing the flag when you launch the gui has no effect.