EDIT: This does not work
I used EasyBCD to add an NTLDR entry for the second parition.
When I rebooted, selecting that entry gave an NTLDR not found error.
I copied NTLDR and boot.ini into that partition, and the entry worked, but still read boot.ini from the system partition.
Warning: Extremely Dangerous!
If I really wanted to, I could open the second copy of NTLDR (in the second partition) in a hex editor and change the references to boot.ini to, for example, boXP.ini. I could then create a boXP.ini in the system partition and only give it an entry for XP.
Of course, if ntldr does integrity checking, this won't work at all.
EDIT: I tried this in a virtual machine, and, when I rebooted into the partition with the modified copy of NTLDR, it said,
NTLDR is corrupt. The system cannot boot.
This wouldn't be as dangerous as it sounds because the actual system partition would retain its pristine NTLDR, and have a boot.ini that defaults to Server 2003 with a very small timeout.
Even so, I'd prefer to do something more sane.