Radial on the rear (non-drive) is doable, but I would not recommend it - especially for a fixie. as others have mentioned, it puts additional load on the hub flanges in ways that most are not designed for, and even though it might look cool, it yields a much weaker wheel that is also more prone to torsional wind up (hub rotating within the spokes). Not wise IMO.

Only lace the front radial if your hub can take it (i.e. is designed for radial lacing).