I haven't adressed HTPC, because honestly calibration there is more complex. You're dealing with a number of different decoders depending upon format, that part I'm somewhat sure about and there are some other items I'm not so clear on. With Blu-ray and HD DVD players basically we're making the assumption that mpeg2, AVC, and VC-1 will be correctly decoded to the same levels by the player. I've seen on my computer though that mpeg2 doesn't always necessarily match AVC levels, so I have to attribute that to the different decoders. Even using the PowerDVD trial, with my ATI card (known to have issues with affecting levels) I was running into apparently different levels based on the source type.
Basically where I'm going here is that it's probably more important on a computer, than a standalone, for the source to match the type of material you'll be playing back. If you're mainly playing back DVDs, then it seems to me that a DVD calibration disk would make more sense. If you're playing back recorded ATSC or QAM which might be HD mpeg2, then you probably want a HD mpeg2 source. The HD DVD here is mpeg2, but the container is basically specific to HD DVD. Generally if HD mpeg2 playback is what you use your computer for, then patterns at w6rz.net might make more sense because they're .ts which will likely be closer to the format of your meda.
If you were running HD DVD and Blu-ray on your computer I'd have a different response, but that wasn't the impression I got from what you wrote. All of these disks do originate from mpeg2 files outside of the Blu-ray and HD DVD formats, but currently we have not been hosting those files and it would be something that would have to be discussed. Honestly though, there are probably tools available to split the HD DVD and Blu-ray videos into more common formats that would play back on your current software if you are trying to calibrate for playing back HD files on your computer.