Yeah, I think you're correct that you would have to develop a custom solution. The menu controls all pull their menu items from the context of the current site. You might be able to use the StartingNodeUrl on the SiteMapDataSource for your menu but I don't think you can pass it a complete url and if you can, I'm not sure the SiteMapDataSource control would be able to pull in the menu items from the other site. If it can pull in the menu items, you might have a solution because then you can create a SiteMapDataSource and mojoMenu for each site and that can be done programmatically.
I'm interested to know how you make out with it though, it's an interesting concept.