Regarding Signal, I don't think they have been approached yet... https://github.com/UnifiedPush/wishlist/issues/8

I think the consensus was that we wanted to wait for UnifiedPush to be more mature before approaching them, but it's probably about time. S1m made an interesting implementation for the (compatible) Signal fork Molly: https://github.com/mollyim/mollyim-android/pull/152

The implementation is quite interesting: it adds a linked device that does not receive encryption keys, but sends push notifications to the client.