The good news is that Steempeak has already made this possible!
On their beta site you can now cross-post onto other communities!

@steempeak is awesome. Filled with talented people as you said. I've been using it for a good while now. Great to have communities also in Steempeak beta. I better start using that too, affecting immediately. And it's a really good thing that OCD curation team is also supporting the smaller communities. Or OCA at least for starters.

Thank you @OCD and @steempeak for all your hard work and extremely quick moves. I appreciate these two qualities very much.