Hey, thanks to you for working on this project! It’s a neat idea and a good way to start dipping into a modular workflow.
The expander protocol is something you build in when developing modules (here’s a quick explanation in another thread), so it’d only come in to play here if someone were coding a new 901A/901B module and wanted to simulate the historical wiring approach. The sound would be the same either way–it’s just a question of whether a front panel cable is used.
I think that for this project it’s much better to depart from the design history a little and use the modern convention of front panel pitch inputs (since that way you can pick and choose oscillators). Rack modules (and modern modular design) would generally never hide a conventional input that way; back panel connections are almost always for proprietary connections between parent modules and expanders (search the Library by tag for examples).
Are you thinking about the purpose of it, or about how to emulate it? If the former, happy to help–say a bit more about what seems obscure? If the latter, here’s an interesting discussion of its particular characteristics by Chris Meyer (which you may have seen already). In particular, a clean mixer won’t get you there, because it distorted pretty easily. A lot of the available Rack mixers are ultraclean or have standard simple saturation at the rails, and I don’t know offhand if anyone has done a physical model of the CP3 in particular. The @slimechildaudio Saturating Mixer is the closest thing I know of, but it’s paid. Building a CP3-like mixer from submodules would be an interesting exercise, actually!