The way you have it drawn has a few issues:
- It's always bypassed when the footswitch is not plugged in
- When not bypassed, your Solo control actually becomes a reverse "gain" for the diodes, at best it can add the equivalent of the diode's voltage to your signal
- The clipping will be very subtle... potentially undetectable.
- The footswitch cable will act as a capacitor that bleeds a lot of highs to ground (this can be a good thing with clipping, but it's a bit of an unpredictable way to achieve it).
Personally I would separate the clipping circuit from the volume boost. I'd move it before the master, with an inline trim so you control how much clipping you get no matter how the volume is set. I'd use LEDs to not remove too much signal, as much as 3 in a row per side if that's what it takes. Then for the Solo control you have it right, the challenge is that it takes at mininum a SPDT switch (alternate between grounding the clipping circuit or the Solo control).
At the end of the day though, a relay, a diode, an LM7805, a bridge rectifier and two caps isn't a lot of $$$ and you got yourself a relay circuit, no need to send audio down the footswitch cable. And you can do something like I described.