There has been something on the periphery of my actual thought process for a while now. Here goes:
There has to be something one can place on the strings above the pickup that will emit a steady test tone in order to set levels by way of a DAW or anything with a real meter in it.
First thought is tiny speaker being fed tone from an audio signal generator. I will try my phone with a signal generator app in a bit.
I'm sure you all can come up with a device to do this and allow us to set very precise levels.
Bueller?!?!
Also someone had this brilliant idea a while back and it applies not only for setting levels but also tweaking sounds. Place a looper in front of the effect which you are modifying so that you have a constant, consistent source without the distraction of playing. (Also frees up the hands and brain)
I think for the kind of level matching being discussed, you could take the bass out of the equation all together and just use a tone generator with some pink noise. You won't be able to hear the sound of the effects properly, but at least for matching master output to bypass levels, this would be the easiest.

