Reverse engineer the Analogue Systems RS-35?
Apparently it can be done:Guitar into RS-35
And with regard to the latching function, as it is a module for a modular synth, it's a matter of adding a module. Filtering and compressing is advisable and can be done in front with other modules (although the module has some bandwidth limiting built in). Apparently, adding a slewing module is also recommended.
Haven't tried the RS-35 module myself, but it does seem a very good candidate (for bassguitar as that's my primary instrument).
I agree it's not easy, and the hexaphonic approach is a lot better. Had a look at the documents you linked, that Roland pickup looks diy-able, do you think it has changed much (wrt. the current GK-xb, not sure which number they're at)?
Not sure how actual this thread is, but I've bought a secondhand Analogue Systems RS-30 pitch to voltage converter, and it uses a LM2907/LM2917 (don't recall which one) pitch to voltage IC.
The power supply in my modular is down, so I haven't been able to try it out extensively, but definitely does not feel like a plug-n-play solution.