Are you quite certain that
VolSnap.sys isn't included in your PE? It is a Microsoft Windows driver. I don't think you need to worry about any VolSnap references at all.
In order to work with (load, modify, unload) a Registry hive file, that hive file must be on a writable medium. So if you have hive files on a CD/DVD, copy them to an HDD filesystem before attempting to work with them.
A Windows XP/2003 PE initially populates its
HKLM\SYSTEM\ hive from two sources:
Items in
TXTSETUP.SIF are loaded by
SETUPLDR quite similarly to how "boot-start" services are loaded by
NTLDR for a non-PE Windows. That is, these drivers are loaded at pre-kernel time.
Items in
SETUPREG.HIV will be initialized at post-kernel time, as far as I know.
Now
I know you've been sworn, and I've read your complaint...It would seem that you might be able to inject the
sbmount item into SETUPREG.HIV, as its
Start type is
1 (system-start).
It would seem that you might be able to inject the
stcvsm item into TXTSETUP.SIF, as its Start type is
0 (boot-start).
Remember to copy the
.SYS files, too.