@ Bezalel, FreeStyler, Severach,
I'll explain what I was doing, which should answer most of your questions.
I was making two OEM Multi-Boot disks, one for Home and one for Pro. Each image included OEMBIOS set, the PID for that set or the Microsoft System Builder PID, oemlogo, info, wallpapers and themes
[if available] 
As I added each image to the disk, I tested it in VirtualPC. Which was setup to dual boot, and had the BIOS modifed with the SLP for that OEM. I know its never a great idea to test two different things at the same time, so any problems I found
i.e. PID failing, wallpaper/theme not displaying I noted down so I could look i to it later and continued on with the next install.
All was fine till I tried the Hedy set. Both Home and Pro images installed fine, except the Home preactivated but Pro didn't. Which was strange as the hardware/BIOS were indentical
[Dual Boot VirtualPC] I rebuilt the Pro image making sure I used the correct OEMBIOS set and installed it again, with the same results.
Now my Multi-Boot disks are complete
[i think]
I'm going to test them on a real system. I'll start with the Hedy installs and report back.