I spent some time tonight taking apart three Shimano hubs (RSX100, Deore M525, and Ultegra 6500) and found some interesting differences. While they will all mount in place of each other, there are some small dimensional differences that could pose a problem.
The most critical difference is what I'll refer to as the "cassette offset". This is the distance from freehub mounting surface to the surface which stops the cassette from sliding further onto the freehub body. The Ultegra hub measures 6mm while the RSX hub is 4.9mm and the Deore 4.3mm. The Ultegra hub is machined noticeably deeper than the other two. Putting the Ultegra freehub body in place of either the RSX or Deore results in an obvious gap between the freehub body and hub. The difference between the RSX and Deore seems negligible. With the Deore mounted in place of the RSX, the cassette is still plenty far away from the spokes.
The seal area of the three hubs are all different. Both the Ultegra and Deore used a 18.2mm diameter dust cap ID while the RSX was 17.5mm. The dust cap height and distance from the edge of the freehub are different amongst the three. Basically, if you find a comaptible freehub from another series, expect to need the cone and any seals from that hub too.
Finally, the Ultegra and RSX hubs both use 33.8mm long fixing bolts while the Deore uses a 36.8mm long bolt. The all gave basically the same amount of actual thread engagement though as the start of the thru hole is counterbored and only acts as a pilot for the bolt. The Deore hub is fully counterbored out past the major diameter of the thread whereas both the RSX and Ultegra hubs are only machined far enough to just let the bolt slip into the hole. You can still see remnants of the thread and it appears as if the thread has been partially stripped. Weird.
Looking at Universal Cycles website, it appears as if Ultegra 6500 and 6600 (no 6700 picture unfortunately), 105 5500, and Sora 3300 all use a similar freehub judging by the cassette offset. 105 5700, Tiagra 4500, and all MTB hubs (that are pictured) except for XTR M985/975/970 use similar freehubs. The MTB hubs (including the Deore I checked) use a different washer behind the freehub though. It's larger in diameter and has the sides cut away. The RSX and Ultegra hubs use exactly the same washer so it seems safe to assume that all road hubs use the same washer.
Freehub with 6mm cassette offset:
Freehub with ~4.5mm cassette offset:
If I had to guess based on what I've seen and the picture of the hub shell in the Shimano tech docs (they don't show the backside of the freehub), you need a 6mm offset freehub meaning any Ultegra hub or the 105 5500 or Sora 3300 freehub (again assuming Universal Cycles' pictures are correct).