DIY Hybrid how?

If I wanted to take something like a Teensy synth and give it an analog filter... How would I do that?

How would one route polyphony? Is there a clever way around having a DAC per voice other than going paraphonic like the Microfreak?

What about filter envelope? Is that another DAC per voice?

And what about output? Is it better to stay analog, or to go back to an ADC for recording or effects?

It seems like a huge mess, which is probably why I cannot name a DIY project that does it. Can you?

Am I missing something?

Analog filter on digital DIY synth - Can it be done?