Can't find it myself, either, and I'm probably pondering a different solution now, some 10 years later, anyways.
Here's what you ought to do, though. Make sure you use a low current/power chip for U3. That could be a TL062, a TL022, or an LM358, among your choices.
You can see a wire jumper situated between R16 and C8 on the parts layout. That connects up V+ to pin 8 on U3. Replace that jumper with a low value resistor; anything from 10R to 47R or so. Run a 1-10uf cap from pin 10 to wherever the nearest ground point is. Naturally, the + side of the cap goes to pin 8.
That will provide some decoupling and isolation such that when the LFO suddenly draws current to produce a square wave, there will already be some reserve current in that added cap, such that there won't be enough overall current draw to produce a spike in the audio path. Plus whatever current it does draw will be lowpass filtered by the added RC network.