Sorry to bump an older topic but I wanted to add my own experience with this issue in case it helps someone else in the future.
My friend gave me his softmodded XBox the other day. The first thing I did was upgrade the hard drive using xboxhdm. I then replaced the dashboard with XBMC and started loading all of the emulators and games onto it from my chipped xbox directly through ftp. The whole time I was doing this I had the new xbox hooked up to a small Commodore 1702 monitor using AV cables. Once I had the XBox all set up the way I wanted it, I brought it out to my living room to hook it up to my HDTV (which is where I had intended to keep it). Only after hooking it up did I realize that the HD options in XBMC were unavailable because they had never been set in the original dash.
For various reason all of the options outlined in this thread would not work for me. For whatever reason I am unable to launch the MS dash from any file explorer, and I do not have that ntds business. This is what I did, and it may or may not work for you.
For starters, my softmod uses that nkpatcher business. I don't know anything about it because I am not the one who modded this deal. I have only chipped xboxes, I have never softmodded. Anyway, the set up I have is done with nkpatcher and a shadowc.
You would need to have the following available to you:
-Frosty's Rescue Disc (downloaded from the usual place)
-Splinter Cell (or one of the other games used for softmodding): Splinter Cell is $1.99 at Gamestop
-Memory Card: $2.99 from Gamestop
-Hacked gamesave files from you-know-where.
-A means of getting the gamesave files onto the mem card (I used my other XBox which is chipped)
I ftp'ed into my REAL C drive (NOT the shadowc) using the evox rescue dash and replaced the patched xboxdash.xbe with the original xboxdash.xbe (after backing up the patched one, obviously). This disables the softmod. Upon resetting the XBox, the normal dash is loaded. I then enabled all HD modes. Reset. I then used Splinter Cell and a memory card loaded with a gamesave exploit to load an Evox dash, turned on the ftp server, and restored the patched xboxdash.xbe in the root of C. Restarted the XBox again. With the softmod restored, it loaded XBMC but I still didn't have HD modes available. When I ftp to my xbox with XBMC running, I see the shadow C instead of the real C, so I had to once again boot into a rescue dash so that I could ftp into the real C. Then I renamed the eeprom.bin file in my C root to eeprom_off.bin (although I think you can rename it to anything else since you are just making it so that the virtual eepron is not used). Reboot, and with the REAL eeprom loaded instead of the virtual job, HD settins were available in XBMC.
Like I said this worked for me based on how my XBox was set up when I got it. Depending on how yours is done, it may or may not work for you.
Chris