So, it was a bit tricky thanks to it being glued in place, both from components being hidden from view and from hot-melt glue obscuring circuit traces and component values, but I managed to trace the EQ board:
I ended up having to continuity test to find one circuit trace, and to discover that the CW end of the bass pot was disconnected (also, when at zero, the resistance from the CW end to the wiper is a precise and steady 100k ohms, so it was not in a circuit), but a strategically placed gob of glue still concealed the true value of the brown capacitor at far right above-220pF is my best guess.
The traced schematic looks odd, but after hours trying to wrestle the SPICE circuit simulator in KiCad 8 into submission (it's pretty bad-a dumbed-down GUI that depends on KiCad models for a lot of SPICE functions such as sources - come back LTSpice, all is forgiven) I worked out that this is actually a functional circuit design, taking into account the 220nF input cap from the main board and the 470k output load to ground on the main board. The midband is centred about 365 Hz and can attenuate by a range of about 24 dB. I believe the EQ (really a tone control) attenuates about 7dB with controls at maximum, which is close to the attenuation from the bypass circuit.
The only gripes are pretty minor ones-design best practice would have been to connect the floating pin 3 of RV3 to the wiper to mitigate pot rotation noise, and the multiple ground connections on the ribbon cable header really should be matched at the main board but aren't (see Part 3).
Just to wrap up, this pedal seems to have earned a permanent spot between the bass and my amp. It sounds that good. And here is the only remaining information about or mention of the pedal on the internet - in the Wayback Machine.
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