The reason iMessage works well is because it's integrated tightly into the OS. So for an open-source alternative to be competitive, it would have to run on... an open mobile OS, which would in turn require open hardware.
Heh? Have you tried solutions like Whatsapp, GroupMe, or Viber? They all work absolutely fine on both big mobile OSes, regardless of hardware.
I don't see the need for absolutely open hardware for an alternative messaging system. Sure, they aren't FLOSS solutions, but they are much less closed than iMessage and are cross-platform.
Fine, but Viber does, right? There are open source, secure messaging systems on the way too. My point was that you do not need open hardware to have a relatively open messaging system. Just look at IRC as an example. It might be somewhat dead now, but it certainly runs on everything now.
Sure, you can use proprietary hardware (iPhone) and try to make it run different software, but your luck getting around their protection systems may vary (and it may or may not be illegal, depending on how much you care).
Curious: has this even been tried? I would assume Apple SoC's require binary drivers, so even though they are similar to Android phones on a hardware level it might be difficult or impossible to even get an Android port to boot.
Hell, it can be difficult to get an unsupported version of Android to work without major bugs on an Android phone.
Important to point out: the authors of iDroid stopped, but not due to any technical obstacle; it was mainly because iDroid was a hobbyist project and their lives intervened. Someone else could pick up the effort today, if they wanted.
However even if someone picked the project up again it wouldn't run on any modern devices, since the last iPhone to have an untethered bootrom exploit was the 3GS.
