From the pic it looks like the best brake option is disc. It looks like V's would come up through the rack area and a canti cable would come down through, not to mention you would need a headset cable guide to run the cantis. I suppose all of that would be fine if you planned on leaving the fork on all the time, but what would be better than a quick conversion? All you need is a hinged clamp front brake lever and then you can rock down to cargo avenue!
The fork retails at $200 plus $25 for whichever brake mount you choose. I really do recommend running brakes. If you're hauling 100 lbs of stuff you don't want to have to rely on your rear brake or your legs!
You're probably asking yourself, "Where do I get a 20" front disc brake wheel?" Recumbents! Or you can just get a disc hub and build it on a BMX rim. You know those jams come in all kinds of colors. Colored spokes, too!
Imagine how much beer you can carry!