On the Echo Base when the modulation level is set to minimum the PNP transistor + 39k combo looks like a very small fixed resistor, I don't know the value but its probably less than 100 Ohms. Providing the PTAP doesn't need the 1k series resistor for timing accuracy or current limiting purposes it will work fine like this. If it needs the 1k for any reason you would need to add this, this would then raise the minimum delay time so the chorus type sounds with the delay time pot on or very near minimum will probably be affected.
Even if you don't need the 1k resistor, according to the datasheet for the MCP41050 (page 14) the minimum resistance when used as a variable resistor is around 125 Ohms, so the chorus type sounds might be affected anyway.
The 1k resistor is taken in to account with the lookup tables for timing and current limiting. I think the delay may not be able to get low enough for chorus. (2.5V / 1mA = 2.5k min total R for a min delay time of about 56ms. Good enough for 16th notes at 250bpm though!)
Does it sound smooth though, because on some digital delays you can hear that the delay time is changing in distinct steps. What I'm talking about here is grabbing the pot and sweeping it back and forth as an effect not just setting the delay time with it, I should have made that clearer when I first asked the question.
Looking at the datasheet and the traces of changing from setting 0x7F to 0x80 I'd say it looks like they didn't bother using a grey-code in there. The resistance bounces around for a couple hundred nano-seconds before settling down. So in theory, no. However trying this out I'd say at human scales it seems to work. It's not really intended for that though.
Just thinking aloud here, I wonder if you could replace the voltage divider feeding the MCU with a CV in and modulate the delay time using the digital pot. That would be fantastic 
Good idea, not sure it would work though. The code is averaging out the input specifically to avoid the situation where the ADC varies around a bit transition leading to unintended modulation. Then there's the problem of the digital pot transitions as well. So at this point I would say that it is not well suited to modulation circuits. However I also was trying to make the modifications of the host delay circuit to use this controller minimal. Maybe some good old analog cleverness thrown in can help out and make the PTAP "Echo-Base Compatible".
A couple questions then for you: What is the min/max voltage swing and frequency of the modulation, and min/max delay times?
