A freewheel, not a freehub (+ casette)
Sheldon Browns site seems to suggest with 2 chain whips you can remove the threaded sprockets off a freewheel. I realise I'm not going to be able to get new ones indivually, but If I could (is this possible on all freehweels? mine is pretty low end), then maybe I could knock up some spacers somehow and re-use one of the sprockets to use a SS freewheel
Else are threaded SS sprockets (to go onto threaded SS hubs) then same thread as (shimano) 7spd freewheels?
instead of moving sprockets to get a chainline, why not use what you have and pick the appropriate chainring to get the gear you want?
The sprockets on a 7spd freewheel will have ramps and low profile teeth to aid shifting, so using one of them as a singlespeed would likely result in the chain being thrown off far more often than using a dedicated SS frewheel or sprocket.
If you use the freewheel intact with all 7 sprockets the worst case would be for the chain to ride up onto the next largest sprocket resulting in a bent or broken axle. I've seen that happen several times.