Cashu, for instance, or opendimes, would work just fine, because you're not broadcasting anything to chain necessarily. Both would require trust at that point, as would any other custodial solution. But give them some sort of a seal that can't be broken, and maybe tie in WoT ideas to add in some tamperproofing, and i think given the circumstances you could arrive at something vaguely acceptable. Still not better than subspace radio, but some science fiction actually tries to be plausible...

As long as all the hashpower is in one place it’s fine, you just wait longer to see the conformations. You can use a local lightning network on the same chain fine. It’s when the hashpower gets distributed that things start to fall apart.

Imagine if all the hash for a galactic currency was tied to one very small solar region.