I have a very similar problem (my use case is wanting to space staves more or less distant from each other on a per-system basis), and the solution I got at the time was something like "insert invisible boxes or \hideNotes expressions into a simultaneous voice to force specific spacing." That seems to me like an ugly hack, somewhat contrary to the idea of separating content and presentation, but I don't think the engraver itself has options to set spacing on such a fine-tuned basis.
It's unfortunate, because I'd really like to be able to do this myself, and it seems like something that could be implemented with some sort of \override.
Cheers,
A