I'm not sure if this is a valid use case since the point of this effort seems to be for 10.8, but I've cloned the repo and have compiled cleanly on my "Mac OS X Lion 10.7.5 (11G63)" system. It seems to work great so far, but the scroll wheel is not behaving correctly.
The symptoms were first noticed in acme where scroll up/down does not consistently work in a window, and often delivers the events to any of the windows in a particular column. This has the unfortunate side effect of having the keyboard focus land wherever the mouse event last went, so typing within a window isn't consistent if the scroll wheel is used.
Testing further in rio looks reasonable at first, but for long rio windows the mouse events seem to teleport up and down by large chunks. Also interesting is a seeming draw oddity in rio terminal windows where long lines that wrap around off the top of the window get repainted inconsistently on scroll-up. As an example, open a new rio window and cat /dev/mouse. Generate enough mouse events to have a reasonable window size. Hit delete, then mouse-up. The result is trippy. Mouse-down and the result is as expected.
I'll keep poking at it, but I wanted to mention it in case you'd prefer to close it as not supported right off the bat.