can a single sprocket be locked on a road bike?
How about a Schlumf drive? 




  The Schlumpf drive is an ultra thin planetary gearing system located at the right end of the bottom bracket, between bottom bracket and right crankarm. Installation of a Schlumpf drive hardly changes [either the] position of the chain nor position of the crankarms.


They're probably expensive, but I don't think they have a freewheel. Plenty of people use them with hub gears for extra range. You tap the centre with your heel to shift - not as easy or quick as a lever on the bars, but they look pretty minimalist as they don't have any cables, which seems to jibe with the fixie way (if I've understood it right?)