Flip-flop freewheel to fixed?

So a sprocket will thread on okay, its just you can't lock-ring it on? So you could rotafix it on really tight and promise not to back-pedal...? (Audax events on fixies allow swapping between two fixed gears, but you're not allowed to swap between a fixed and a free - doesn't sound worth the hassle/risk though).

No point really. You could rotafix it on really really really really tight and it is still going to come loose if you apply back pressure to the pedals. New hub+spokes+build is not very much money, especially if you build it yourself or ask someone from here to do it.