I may or may not have invented something...

[genocidal tendencies]

Power attenuator pedals. (forgot to mention, they would be expression pedals) Basically one would allow you to use volume effects without your amp cleaning up. The tone of your speakers will be compromised slightly when you reduce the power a lot, but it shouldn't be too bad. What I think would be the main use of these pedals is in this sort of rig:

guitar->pedalboard->signal splitter->first head->attenuator pedal->cab
There is nothing here at all. Nothing->2nd head->attenuator pedal->cab

It would be an obscenely expensive rig, but other than that, if you used two identical half (or full) stacks, and place the pedals so you can use both at the same time, you can manually control fading effects, and have your drawn out harmonics wailing from one side of the stage to the other.

Anyone seen anything like this before? Judging by my history of coming up with new ideas they probably predate electricity.

[Searcher]

I haven't actually heard of anything exactly like that, but can't you do similar stuff with many multi-effects pedals with stereo outs and the expression pedals?  I know it's not the same though.

[FELINEGUITARS]

Sounds not a million miles away from this pedal
Basically it drops the output to a rythym volume and then you can unleash the full output of your amp for soloing.
It is a preset that YOU set but a similar principle
http://www.award-session.com/solobooster.html

[genocidal tendencies]

^I think the main difference there is that it's not an expression pedal. Reading back over my first post I somehow managed to not mention that I meant expression pedals... Oops.

^^you can, yeah, but that's before the amp, so if you're playing anything heavily distorted, it won't get quieter, it'll just go clean. Of course, you could use the distortion of the multi FX into a clean amp, but that probably won't sound so good. Also I love the idea of using two expression pedals at once. I was wondering a while ago what two wahs would sound like...

[Searcher]

I read somewhere that SRV used two wahs at once for Say What.

[HJM]

Matamp made an attenuator pedal to go with the C7 Heads, dial in the rhythm volume on the attenuator and bypass for your lead tone, worked pretty well.

The Award thing is a volume pot in your effects loop, giving dual master operation. Good trick but £40 for a pot in a box???

A remote box with a relay would be my way of doing it, then you don't have speaker cables everywhere, you don't want a speaker lead getting pulled out by mistake!!