So I just tried this and I think it has potential. Using an old faulty iPhone 7 running AUM, and a Behringer UCA222 I was able to implement about 30 parametric EQ instances in AUM with around 30% DSP load. The latency between that system using FM and another was inaudible. I think with the right AUM plugins and a multichannel interface you could do crossovers no problem. I'll keep playing.
If I understand, you use iPhone as DSP with sound card via USB (camera adapter?) for input and output.
Martin
Yes that's it, pretty simple. The UCA222 USB sound card handles the input and output. AUM app handles the DSP. It has its own parametric and shelving EQs, but can also host iOS AU processing plugins, of which there are many.
I've ordered a cheap battery connector adapter that is normally used by iPhone repair shops for testing that will allow me to remove the battery and run the iPhone from a DC power supply.