did you ever make this work? There's also a new version of LDprep that you can run from within the OSD script. Have you tried that?
The "old" usage mode, whereby you ran ldprep after sysprep and the driver enumeration happened first thing after the reboot into mini-setup, isn't going to work on a 64 bit OS.
As to the "new" usage mode, whereby you run ldprep under WinPE, I can see no fundamental reason why it wouldn't work with WinXP-64, but I haven't tried it.
"New" is a relative term here. The new usage mode has been the normal usage mode ever since V8 of the HII whitepaper (March 2007).
There have been a few minor tweaks to ldprep after V1.5 (current version is 220.127.116.11 - included with V9 of the whitepaper), but none that could possibly make a difference in terms of WinXP-64.