Since the PowerQuest image format is just a dumb bag-of-sectors (essentially identical to VHD/VMDK/VDI, using a similar kind of compression to that supported in VDI) it's actually supported both through the regular Ghost client and I believe through GhostCast.
Basically, because these formats are all just dumb sector bags, Ghost contains code to mount V2i and VMDK as if they were regular drives, and then uses a disk-to-disk clone on them since they are structured *as* raw disks rather than archives (as .GHO images are).
In the case of GhostCast this is a little tricker, so what GhostCast does is actually *launch a copy of the Ghost client* when you ask it to distribute a VMDK or V2i image, and this hidden copy of Ghost then does a disk-to-image clone into a named pipe, which the GhostCast server then distributes as a .GHO-format image to the Ghost cloning clients.