Regarding ENV->LFO, you have to decide whether you want one ENV to affect your one LFO (which is then applied universally to all voices), or if you somehow want the ENV to affect how the LFO is applied to each individual voice independently based on that voice's ENV.
If you want it to affect each voice inidividually, that will be much harder. I think that you'd need to introduce a VCA to control the LFO amplitude going to each voice, or you'd have to implement a single VCA that would be in-sync with the CV multiplexing that is going on within the Polysix. Both sound too hard.
If you don't mind your ENV->LFO mode being applied to all voices, then you can think of it as a mod that is mostly useful in unison mode. In unison mode, all the envelopes trigger at the same time. So, you only need to tap into one of the ENV signals. Pick any of Pin X0-X5 on IC24 on KLM-366.
Then run it to a pot or something so that you can attenuate it.
The hard part is injecting your signal into the LFO (MG) generation circuit on the bottom right of KLM-367. The existing MG AMT control voltage is mislabled on the schematic. It's the one that comes off Pin X5 off IC19. Ideally, there would already be a summing point somewhere in that pathway to which you could add your ENV-based CV. Unfortunately, there is not. So you might have to add an op-amp that is configured as a summing amp, pull the existing CV off KLM-367 (I'd break into the path by snipping out R78...take the signal from the upstream leg of R78), and sum it with your new CV from the ENV circuit.
From the output of your summing amp, you'll have to add some resistance to replace the impedance of VR2 and the R78. After adding that resistance, drop your signal back into the path on KLM-367 (drop it into the downstream leg of what was R78).
Assuming that the DC levels of the two circuit paths are compatible...this should rock!
Chip