Depends on whatever crazy block output the connecting chain has.. If it's something machine readable in a consistent and known format that can be spat out then it's pretty easy to pick up with another system.

Hell, it wouldn't be impossible to have a smart contract on HSC to do nothing but act as a gateway or access point for other sidechains.. That is the beauty about doing it the custom EVM route.

Hell you could use any ledger that these sidechains can get data out to really. even something like IPFS could be used to send stuff across chains. As for natively supported alternative tokens going to have to see what I can come up with.. have a few ideas but won't know for sure if they'll work as planned till I have a whack at it.