Right -- I think I could see where you were going with the "42 in a 48 in a PDA" idea. Sorry to treat it so flippantly. (But I did like the notion of seeing how many layers of emulation could be experimented with!)
I've tried EMU48 and found that the slavish implementation of a "virtual keyboard" and tiny simulated screen to be prohibitve limitations. These are made all the worse in my case because I've only afforded a monochrome PDA, and so don't get the benefit of colors in the interface. The horizontal pitch on the iPAQ is not quite enough to double that of the HP-48's screen, so the developer is limited to mapping them pixel-for-pixel. Consequently, the emulated screen is really miniscule!
So, I think you'd end up stuck with the too-busy HP-48 keyboard simulation, and the 42S output crammed into a microscopic version of the 48's display -- not a good situation.
I have expressed a hope that I could someday get 'round to utilizing the EMU48 (or similar) code as a behind-the-scenes "calculator engine", and somehow translate the PDA's standard input into virtual keystrokes. But such a project is simply not in the cards right now, and I've enough HP-32S/SII models stashed at the office, in my car, and in my toolbox that I don't really need it . . .