>I'm more interested in how those layers could be used with my own engine than theirs itself.
You and me both. Also looking at Proton [1] and Allegro [2] for similar reasons. And just came across a Blackberry-backed engine [3] which looks interesting (and also Lua-driven).
I actually have my OWN game engine (based on LuaJIT, thank you ;) that already targets Windows, Android, and iOS, but I'd like to rip out the platform layer and replace it with something that someone else is maintaining. I mean, it works now, but leveraging open source and other developers' efforts would be much better.
I've considered just using SDL 2.0 and calling it good, but I'm tempted to pick up some of the higher-level functionality some of these other engines offer.
None offer all the features I want, unfortunately. One of the big features they mostly miss is really good script/C++ cross-functionality. Aside from that, there are typically one or two other things that make me want to avoid them.
[1] http://www.rtsoft.com/wiki/doku.php?id=proton