The team focused on internal use though, and hasn't had a chance to open source the backend and flesh out the web side of things. I hope they get a chance to do another iteration (we were bought by Automattic/WordPress and I'm no longer there).
[1] https://github.com/Simperium/simperium-ios
[2] https://github.com/Simperium/simperium-android
I think a large part of the reason such standalone libraries might not exist yet, is that it's really difficult to make a general, flexible, tech agnostic solution to this problem - as you've highlighted this is probably why Meteor's implementation is so tightly coupled with Mongo and why SQL integration is taking a while (assuming, surely, it must be a pretty high-priority feature on their roadmap).
In other words such a library is likely to be so prescriptive on how the rest of the stack, particularly the backend, fits around it that it only really makes sense as part of a full-stack framework, or at the very least a prescriptive full-stack architecture.