I’m pretty sure the P12s inputs are the same as the Rev2, 5V like the EP3 pedal, which is why i find it behaviour.
As for using DC, I don’t how I’d use that to make the pedal extend its influence on the filter. The pedal is not a destination in the matrix and if I apply DC to the filter it’s really just going to be the same as opening the filter more right?? (Assuming a + value of course)
***update, never mind, I can of course set DC to a mid slot, forgot about that.
On another note you can set the pedal inputs on the P12 to filter via the global menu too, and then you have the option for half range and full range, so can achieve the same effect that way.