I actually just ran into this problem as I wanted SP3 on my V90Ls for security and the improved RDP client (yay true dual screen support). As my units have 512 flash drives I could not use the official Wyse image, so I set about making my own.
Process is fairly simple, but you do have to slim an XP install down by about 2/3. I started with a XP Pro SP3 image and used nLite to strip out just about everything, leaving just the essentials. Then I used nLite to integrate the VIA Video, Net and audio driver into the install (CN700,Rhine, Vinyl AC97 respectively). Finally I added KB969084 (RDP 7 update). All said and done my installer is 130MB and after install uses about 400MB, that leaves room for a 50MB page file and a bit of spare space. Granted this wont work well if you intend to add anything else to the system, but for use as a RDP thin client its great!
EDIT: I made a USB boot drive with my XP install image and ran the install from there. Hit “P” at boot to get a boot device list, or use password “Fireport” to get into BIOS setup.