The outside dimensions of the 12ga USH frame are the same and a standard forend will work with it, but not the barrel as you know is much bigger. The difference is the barrel channel in the frame, the USH barrel is ~.100" larger at 1.2", a standard barrel is 1.1" at the barrel swell. If SMI doesn't make the barrel swell the same size, it's not going to fit very well and will be unsupported on the sides. But if they make the barrel to the same contour as a standard barrel, a regular forend will work fine, the USH uses the same forend spacer. Depending on how they taper down from the barrel swell, you may have to relieve the wood some there.
Tim