One of Deaf Eddie's Fat-o-caster switches maybe? May not be what you're thinking of but sounds similar. The TBX is just a dual pot that acts as a regular 250k tone until it's halfway and then progressively works towards no load between 5-10. The Delta Tone control just has a notch at the end of the travel that takes the tone pot out of circuit. I have a TBX in my strat and a DTC in my tele. They both work as described.
http://deaf-eddie.net/