Hi Phil, I have a 2in Dielectric WO diagonal that I got as part of the Zenithstar80 package from Frontier Optics (link below to their site). I had asked Daniel at Frontier about the adaptor so this could be used in a SCT. From memory it was $80-90 so it might be any cheaper than going for the SCT version. If you were interested in an in line focusser for the SCT to avoid mirror flop etc the fitting of this to the SCT thread also acts as an adaptor allowing you to then fit a standard 2 inch diagonal. I was considering the standard WO SCT craford focusser which Frontier Optics are seeling old stock of (now the dual speed version is out) for $220 AU, but ended up going for the Meade Micro-focusser to get the electric focus. This works a treat for me, but I don't have a fork so can't comment on whether it has clearance when the scope heads for zenith.
http://www.frontieroptics.com/Diagonals.html
cheers,