If you had enough threads to add a lockring (BB lockring, not track, obviously), then with brakes this would be a safe setup. Rotafixed cogs, especially with some loctite, are extremely hard to unscrew, but it is somewhat possible, hence the brakes.
This is what
Sheldon Brown has to say about the issue:
Suicide Hub
A rather alarmist and silly name for a freewheel hub used as a fixed-gear hub.
Any standard-thread freewheel type hub will also accept a fixed-gear ("track ") sprocket . This is a common technique for converting an older bike to fixed gear on the cheap.
Despite the silly name, this is no more dangerous than using a freewheel, as long as you keep front and rear hand brakes installed.