The people who wrote to lace up the left side only are right - this is definitely the way - we're not talking theory here, it has worked many times.
The only thing I have to add is: You don't need any crossing spokes, and the length is unimportant. Just grab 18 spokes (or however many fit the left flange), any length so long as it's longer than the radial, and shorter than the cross-4 length. Thread 'em through hub and rim radial style and hand-thread nipples on all. The "wheel" is totally loose at this point. When you go to loosen the freewheel, the rim winds up until all spokes are tight, all are pulling - not just half as in a crossed wheel.
I think theoretically a freewheel could be on tight enough that the hub barrel between the flanges could twist, ruining the hub. I have never had this happen though in the 10 or so times I've done or witnessed this procedure.
Good luck; let us know how it comes out!
Mark Bulgier
Seattle, Wa USA