I had a good look for a library that does this for the app I'm currently working on, before giving up and creating a custom solution myself. My implementation is very much custom-fitted to my particular backend api and the way I'm rendering the view layer with React.
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.
That's basically my take on it, and we also have a bunch of custom code to do similar things piecemeal, but it doesn't work very well and I envy Meteor's solution, while remaining unprepared to jump onto the framework in its entirety. But lots of "really difficult" things have been accomplished in the past, so I'm still hoping to come across a "general, flexible, tech agnostic solution to this problem" at some point!
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.