Got an email from wabjxo that I might be able to help here.
I've made a dozen or so single-layer DVDs (4.7GB) containing HD material (1080i), not downconverted to 480i, playable (only) on HD DVD players.
I actually learned how to do it thanks to a thread in another section of AVS Forums, which I can't find now.
It goes something like this:
1) Record HD material onto HDV camcorder (or you could have another source of HD material of course).
2) Transfer the HDV material from camcorder to PC using HDVSplit (an HDV capture tool like WinDV is for DV material). This results in a *.m2t (MPEG-2 Transport Stream) file.
3) Convert this *.m2t file into an MPEG-2 Program Stream (*.mpg) using tools like VideoReDo or Womble.
4) Import the *.mpg file(s) into ULead DVD Movie Factory 5 Plus, which can author HD DVD folder/file structures suitable for burning to either HD DVD or standard DVD discs. No re-encoding of the imported *.mpg material is necessary for this.
5) Burn the resulting HD DVD folder/file structure using Nero 7. There are some settings here that are critical to making it work. UDF, and Xbox Compatibility Mode off the top of my head.
Apparently, of the two High-Definition DVD standards (HD DVD and BluRay), only HD DVD supports recording/playback to/from the old DVD media (single and double/dual layer).
If that thread could be found that describes the process in great detail, a link to that would suffice. But it looks like there has been some rearrangement of the Forums lately, and that thread is no longer sticky at least, completely gone at worst.
I recently tested the upper limit for how much HD material would fit on a single-layer DVD, by calculation it is 24 minutes (using the 25Mb/s bitrate of HDV), and sure enough I was able to fit exactly 24 minutes with very little room to spare.
(I guess the original question of this thread is just "is it possible", so maybe just "yes" would have been sufficient?)