Hi Shane,
I think Patrick's assessment is pretty much correct. It would probably be possible to manually submit to a cross-domain list by setting up a data connection with CAML and the things involved in that (as we've shown in blog posts and webinars in the past), but the built-in list submit functionality assumes submitting within the current SharePoint tenancy so you wouldn't be able to use the SubmitToSharePointList command for this.
Please let us know if you have any further questions or if we can assist you with anything related to this.
Jimmy Rishe / Software Developer / Microsoft MVP
Qdabra Software