OK, I have put together a short list of what I've implemented so far in RPath, which I'm now going to call ReoPath, to give it a less generic sounding name.
Currently, it can be called from within Mozilla using an XPCOM component, or can sort of be used as a standalone utility. I'll provide the code and/or binary soon after I clean up some crashes and all those memory leaks.
Feedback is welcome.
Next, I'll implement some additional functions. I'll also post some info about ReoPath Templates, which is a way to embed ReoPath expressions in a XUL file (or any XML file for that matter), and have content generated, kind of like XUL templates, but more powerful (and at the moment, with more hacks).