So interplanetary, yeah one planet is going to be the central transaction handler and everyone else is going to have to deal with it. So likely things that require transactions, you'll have to have enough cash in the bank in your Mars account if you want quick transactions, and expect wiring money from Earth to take a while.

Coming back to the current day, if you need low-latency global transactions just here on Earth, then that's probably how you'd have to design it. You prime each region with a certain account "limit" of whatever it is you're transacting to use in local transactions, and when that limit gets low you transact some more from your central data store back into your regional account, or something along those lines. It'd be a two-stage thing.

I remember being very impressed by Spanner when I first heard about it, but as you can see in the OP, it does make a lot of consistency guarantees through sheer brute force. It simply throws power/resources at problems which would generally be considered impractical when normally designing a distributed system. An approach which does of course have its limits (not to say it isn’t still impressive, it’s a great system).