Can I use PyPy and RPython to compile smaller parts of my Python program?
No. That would be possible, and we played with early attempts in that direction, but there are many delicate issues: for example, how the compiled and the non-compiled parts exchange data. Supporting this in a nice way would be a lot of work. PyPy is certainly a good starting point for someone that would like to work in that direction. Early attempts were dropped because they conflicted with refactorings that we needed in order to progress on the rest of PyPy; the currently active developers of PyPy have different priorities. If someone wants to start working in that direction I imagine that he might get a (very little) bit of support from us, though. Alternatively, it’s possible to write a mixed-module, i.e. an extension module for PyPy in RPython, which you can then import from your Python program when it runs on top of PyPy. This is similar to writing a C extension module for CPython in term of investment of effort (without all the INCREF/DECREF mess, though).
