Are you sure it's fixed (does it have the reverse threaded section for the lockring)? I don't think there is really such a thing as a fixed BMX hub. You could definitely build it up into a nice strong wheel. I built up a 48h rear wheel for a polo bike with a DK BMX hub and it's super strong.
BMX hubs have similar spacing to track hubs, have long axles so can tolerate being re-spaced, many of the newer ones have good quality sealed bearings, and many have high flanges. The flange-to-flange spacing should also be similar to track hubs. I say build the wheel, you can make a bombproof wheel with the BMX hub to convert your bike to single speed.
One last thing, if the BMX hub has oversized axles that are not slotted, you might have to file them.