I'm about 2/3 of the way stuffing the board I etched the other day.
I dug my own unit out of the basement, and dickering with it just now, several things are clear to me:
1) It really needs a sensitivity control. The unit is NOT an envelope-controlled filter that corresponds directly to picking strength, but rather employs a triggered envelope. Triggering is not entirely independent of input level. So something needs to be adjustable around U1.1 to allow it to retrigger reliably with both higher input levels, and lower ones. You will note that R3/C1 roll off the bass sensitivity of that stage around 1500hz. Changing the value of R3 to adjust the gain monkeys around with that rolloff. So, in the absence of any strong rationale for using it, I'll suggest replacing R1 with something like a 250k variable resistor in series with 100k fixed resistor, to vary the gain of the stage between 22.3x and 75.5x (stock is 58.4x)
2) A filter range switch is useful. On mine, I installed a 3-position toggle. I replaced the stock 2200pf cap (equivalent to C22 in the redraw) with 2200pf and 6800pf in series. The switch bridges the one or the other or maintains them in series, yielding three ranges. Thinking it over I should have probably used 3300pf and 6800pf, since the highest range when using 2200 and 6800 is useless. Putting 3300 and 6800 in series gets you 2200pf, which returns a stock sound. The unit sounds really nice on lower ranges.
3) We know a transient is generated, and that two versions are produced: a rising and a falling version, equal in amplitude/sweep, and cancelling out when in 50/50 mix. What we don't know - or at least I can't for the life of me figure out from just looking at the drawing - is where that transient is produced. It would be nice to know a little more about how the time constants are determined. Being able to vary the rise or fall time would be nice. I'll haul out my scope later tonight and see if I can spot where appropriate intervention points might be, as well as learning a little more about the transient that is normally generated.
4) The Range switch is also a quirky one. It sounds like it selects between two resonance settings. It could really use a 3rd setting in between the two provided. The stock arrangement provides a high-resonance setting, and another that is so bland it deters use.
I don't want to overcomplicate it, but this could be a much friendlier and more maleable filter pedal with just a couple of tweaks. If it had variable sensitivity, the stock up/down control, a 3-position filter range switch, a better selection of resonance settings, and a 2 or 3-position toggle for fast/medium/slow sweep, it could be a really nice pedal. Let's get this done. Meanwhile, I'll continue stuffing my board and see if it fires up.