Playing with the Windows Boot Configuration Data store is not a task to be taken lightly. It might be worth seeing if the free version of Macrium Reflect can fix up the Windows XP boot, but you would probably find that Windows 10 would then no longer have a way to boot and would have to be fixed up as well. I wouldn't recommend doing any of that without first saving away an image of both partitions.

rknichols - I've switched back and forth between the XP and Windows 10 boot loaders during my testing, I can do it with my eyes closed now,
using the XP and Windows 10 boot media (XP, recovery console, fixboot. Windows 10/startup repair)
The problem is exactly as you said, if I put back the XP boot loader I can no longer boot Windows 10.

With the Windows 10 boot loader installed I can certainly boot XP and it starts up fine, but I get an occasional crash in some applications
that only occurs when XP is booted through 10's boot manager. Firefox was one of the problem apps and one of their developers working in
the "bugzilla" forum have corroborated my findings concerning a crash when XP is booted through the Windows 10 loader on my hardware/dual boot
scenario, specifically AMD FX 4300 CPU

This is why I persevered with this quest; to see if there was a way to boot XP through GRUB, when XP's boot sector was not "installed".

If I find out later that it's possible I'll certainly come back here and update the thread. For now, I can certainly boot XP from a floppy
and leave Windows 10's boot manager installed

I was able to get this to work from looking at the existing grub.cfg, and info I was able to cobble from the internets.
I don't understand all the details but the Grub boot loader branches off directly to NTLDR, doesn't even need the XP boot sector.
Here's the entry:

menuentry "Windows XP" --class windows --class os {
insmod part_msdos
insmod ntfs
insmod ntldr
set root='hd0,msdos1'
search --no-floppy --fs-uuid --set=root DE3C91D93C91AD51
drivemap -s (hd0) ${root}
ntldr ($root)/ntldr
}

For more info:
http://askubuntu.com/questions/13527...dows-partition
https://www.gnu.org/software/grub/manual/grub.html