this is a pretty good idea since the varitone is far more practical than a normal tone knob anyway. a tone switch is just more practical than a knob. there are only like 3 positions anyone uses on the tone knob anyway, 0 (off), 7-8 (a little bit), 10 (all the way). so why not just have a switch that just goes to those settings? also a switch gives you the ability to completely bypass the tone, unlike a pot that is never really totally off.
Surely a Varitone does a different job to a tone knob? If it has an inductor in it (which it should, other wise it is a tone switch) then does it not give a notched filter sort of a sound?
There are a few people online who make varitones. Given that it's basically fuck all in terms of parts, and maybe twenty minutes work, if you can flip that for twenty quid then you're going to do doing well.
I think I'd want a tone knob as well, though. They do different things, like.