After some reading, fiddling and testing I have a working Windows XP Professional with Service Pack 2 for Lenovo PCs installation CD. Here's what I did.
I started with a Windows XP-Pro-SP2 image that I downloaded from MSDN. Using nLite I extracted the CD contents to a HDD folder. I replaced the following files with the ones from C:\I386 on a factory imaged (XP Pro) T61. SETUPP.INI OEMBIOS.BI_ OEMBIOS.CA_ OEMBIOS.DA_ OEMBIOS.SI_ Then I used nLite to create the .iso file and burnt it to CD. When installing from this image I used the ProductKey from C:\I386\UNNATEND.TXT. That product key is a royalty OEM key and along with the oembios files enables system-locked preinstallation. It means that during setup Windows will check that the PC is a Lenovo and then complete Activation without needing anything else.
So now I can do a legitimate bare installation of XP without needing to call Microsoft to activate! Note that it is only legitimate if the machine has an XP Pro COA label stuck to it or a Vista COA that entitles downgrade.
I have tested this disk on both a ThinkPad T61 and a ThinkStation S10.