Given your misunderstanding about how it works, I won't be lining up to buy your widget, but good luck. It's only tooling, pally, no electronics, as you will see.
The indash controller connects to the tuner module, which houses the analog preamp and the mux IC for switching. The tuner receives analog audio from the tape deck (if present), from the indash Business CD (if present), from the AM/FM tuner, and I believe also from the Sirius satellite receiver, and sends that audio stream to the DSP amp.
The DSP amp also can take in SPDIF digital signals on a coaxial cable, but most X5 units made after 2003 seem to be missing that bit. Those send analog audio to the tuner module as well. On most X5s the tuner module is under the spare, but we saw a 2003 today where it was in the dash behind the CD player fascia.
Anyway, the DSP amp has an analog input section where the signal selected by the tuner module is converted to digital. It also has a digital input.
All the DSP functions are performed INSIDE the DSP amp and controlled by the OEM HU via the I bus.
If the DSP amp is removed, the tuner module loses any ability to control DSP functions, and they eventually disappear from the radio screen. But if you'd read and researched your question before posting, you'd probably have found that
Given that, I'm fascinated to see what you make.