Hi guys!
Just explained our official position here:
http://forum.x-cart.com/showpost.php?p=377638&postcount=2
As for uploading custom modules to SaaS store, we cannot allow that, because it is security hole and because merchant can break the store in a way that he/she would not be able to recover it yourself.
Technically, merchant can become X-Cart partner and upload modules (with verification from our side) only for their stores.