I'd like to try this...Would the IRF520 Motorola N-Channel MOSFET do the trick?.....Do you think you could whip up a quick drawing showing exactly where the legs of the transistor should connect?
I was initially thinking this...
(I couldn't be bothered knocking it up neatly like art - I'm a back of a ciggie packet kind guy at heart...and I don't even smoke which means every time I have an idea I have to go & buy a packet of 20 cigarettes!)
...but upon reflection it'll probably 'click' (due to bleed through of the 555 driving waveform & the intercapacitance of the gate/drain)....better going with photo fet like the H11F1
the idea behind it is that mosfet acts as a on/off switch (in sympathy with the 555 driving waveform)...when it's on (gate hi) it's essentially a closed switch and shorts the signal to ground (just like a kill switch), when it's off (gate low), the pseudo mosfet 'switch' is open allowing your guitar signal to pass onwards unfettered. The end result will be a very aggressive choppy signal (i.e. changing from 0% guitar signal -> 99.5% signal-> 0% guitar signal)...but like I say, unless you go with the photo fet, you're likely gonna get click-age!