^ Why not a Mosfet?
I'm not sure it would make a difference from the BJT, really ... seems like a waste of the transistor. FETs just sound a little different when distorted, that was really my only reason for thinking about using one. But to be honest, the biasing will become a PITA.
This is the schematic I'm working on so that we don't have to rely on my ASCII art, might be a few things that aren't correct, but it's close enough. I've more or less decided to go with the MOSFETs for the clippers, though I did want to try the rubber diode as well. The MOSFETs have "simpler" going for them atm.

I'm not really entirely certain how to figure out the exact signal size coming out of Q3 to figure out where the clipping threshold needs to be set (to avoid just squaring off even the low gain stuff) except to build it and measure it. My best guess is:
-The total dynamic range of a 5457 running on 18V is +/-7.5V
-The 10K+47K divide with the 100K tone pot to knock that down to about 3/5, so 4.5V at max gain, except that that changes when the tone knob is set to max.
-Q4 should have a voltage gain of ... ___? Erm, it's 4x based on the 20K/4.7K, but I don't know how to figure in the base resistor divider. I can tell you that the base is sitting at about 1.5V, the emitter at about 1V, and the collector at 10V with a 2N3904.
Honestly, that amplifier seems like complete overkill to me (and makes the output OMFG LOUD even with the 50% divider before the buffer), but no amplification at all means that the MOSFETs barely clip, and decreasing the collector resistors/increasing the emitter rebiases the transistor closer to cutoff, which I don't really want. I guess the only thing that leaves is the 470K feedback resistor to reduce gain? I would think 2x would be plenty of gain there. I'm not looking for fuzz.