Conversions
As one who ( gladly ) invested in HD-DVD movies due to the superior features, I now am in a similar boat.
What I have looked into was converting HD-DVD disks to BluRay discs.
While there are solutions out there, they all seem to be Windows Centric, mostly for the decrypting that needs to happen. One step in the process requires ripping the movie to a independent file.
I would imagine that at this point, you can then convert the ripped movie file to H.264 for AppleTV.
Again, all of these guides are for windows, but if you google "convert HD-DVD to BluRay" you should find the links on several web sites.
Max.