In addition to the problem of obtaining a supply of the food sponge, assuming it's known, a major problem is actually identifying the sponge correctly in the field. Some of them look very similar, and can only be separated by microscopic differences such as comparing spicules. They may look the same, but many of the nudi's only feed on a single species, and they can apparently tell the difference even if we can't. Another problem, as Dr. Rudman once pointed out, is that there is a worldwide shortage of sponge taxonomists, so even if you have the food sponge in hand, it may be difficult to get it identified correctly. Assuming you succeed, you're back to the problem of getting someone in the field to accurately identify the same sponge and collect it for you, which is a fairly tall order for most of the guys that collect for the aquarium trade. It might even be a tall order for the experts.
Cheers,
Don