You can move measures to different systems and systems to different pages in Engrave mode. The idea of having the alto enter at the end of (or middle of) the previous system, is a good workaround for now. I know (see above) that they have fixed the grand staff issue for the next update (promised later this month). They know that we need to have show/hide system control on a page-by-page basis, as it is an aspect of professional engraving. So, that workaround should work.
If it doesn’t work for obvious spacing reasons, you could force a note in the staff that Dorico wants to remove, and then mute/hide the note, using the properties (make it 0% opaque). Unfortunately, you’d have a blank measure without a whole rest.
I assumed that forcing a rest would also work – but Dorico seems to regard forced, entered rests as still meaning the measure is empty when it comes to removing empty systems.
If you use the hidden note trick, and try to force a whole rest in voice 2 (would have to be 4/4), Dorico spaces it to the far left of the measure and up into the space above the staff. As of this moment, the move position property doesn’t seem to do anything.
Dorico will be great when all of this is sorted out. Given the care and thought they’ve shown so far, I have every confidence this will be a king of engraving programs – once the final details are worked out. For me, based on what I’ve seen so far, I’m guessing Dorico will be useable in most situations within 6-12 months. It is already very close in some limited situations.