Morley made a treadle controllable single effects loop
I had it briefly. I wasn’t too impressed with the sweep. It felt more like an ‘on/off” kinda sweep.
Z. Vex makes the Loop Gate
Again, just a single effects loop, but it can be envelope controlled.
If you don’t mind the modeling world, the new Boss GT-1000 can blend between two parallel effects/amp/cab chains based on envelope, and even frequencies. It does have two external effects loops too, so you could put your own out board gear in the chain for maximum tomfoolery.
Speaking of frequency dependent effects chains, there are a few out there I’m sure, but the one that’s caught my eye the most is the KMA Tyler.
Perhaps they could be persuaded to modify the Tyler’s design a touch to focus more on envelope/expression control rather than crossovers.