I have been experimenting trying to get my guitar to work with eurorack, and I have to agree that it's quite difficult.
Essentially it requires setting up the patch first, and cleverly, so that you can control the rack with minimal effort when it's time to record. I use the Planar2 to switch between settings, as I can bump the joystick very easily. I also use Ears as an envelope follower, and the CV from that gets patched into Felix (which is a buffered-mult with attenuvertors on each outputs), which sends the expression basically to all the parts of my rack.
Well so far I've not made anything musical out of it yet... basically setting up takes too much farting around with the sound design and tuning and things. Tuning your guitar with the eurorack is a PITA. At this stage, I'm learning this totally new workflow of doing all the knob-twiddling first, fine tuning the range, and then linking it up to controllers so I can use my feet to control the synth.