I tried a couple of @Galapagoose's suggestions out with various routings on Cold Mac today with some excellent results. Firstly, sending the formant out of the modulator into left on CM and then having the left and right outs going to barrel and formant CV on the carrier, respectively. Subtle changes between 11 and 1 o'clock on CM can have quite a drastic impact on the timbre, when the modulator's signal is being sent to both barrel and formant simultaneously. It's definitely more subtle than FM, but it is really easy to create nice slowly evolving drones.
I also tried the same idea, but going into OR on CM and using the OR & AND outs. Sending the square from the modulator to air (attenuated!) on the carrier in addition to this gives some really nice sizzle. Sending the other outs from Cold Mac to various parts of the patch – Sisters FM, modulate barrel/formant/air on the modulator – and then sending a slow, smooth random (I used R-Flux on the URA) to Survey was delicious.
Sending the modulator's formant out into slope/crease and then taking the crease out to formant CV on the carrier was also really interesting whilst modulating parameters on the modulator, because the audio drops out below a certain level. V glitchy at the lower end, where the audio drops in and out.
I find modulating Survey in a patch like any of the above with Just Friends (or any unipolar modulation source) gives me really good results, because you can set a "floor", of sorts, where the sound of the carrier basically bottoms out (in my case this was the bottom of the range I wanted to reach in the pitch divisions). I've also found with Just Friends that, with a bit of self patching – particularly from some of the inner Ns being sent to ramp, curve, or intone – you can create really slow, organic and evolving cycling envelopes which are great for fading modulation or audio in and out within a patch. I realise that sounds a bit basic, but it's one of my favourite ways to use Just Friends!
Anyone else got any pearls of wisdom?