Would it be possible (is it already possible…?) to select notes on more than one stave to input an independent time signature?
It’s not already possible. You can Alt-click one independent time signature to other places, and you can use the Duplicate to stave above/below commands.
It’s not trivial to implement, unfortunately. We are planning to do some work in the area of allowing more things to be created on multiple staves (only slurs, dynamics, and things you can set via properties, like articulations and jazz articulations can currently be created across multiple staves) but it’s not something you should expect imminently.
At least I’m not just missing how to do it… Only a small thing, but a timesaver.