Sorry for not taking the time to read up on RSAT but are you telling me it is not supported or even available on Windows XP natively? If that's the case you might as well give up getting it to run on WinXP. Sounds like it's using Win7 components we most likely are not able to get up and running on XP. Just like we will not make IE9 to run on XP..
You should have a fair chanse getting one and the same package running on both Win7 and 2K8 R2 though.. As long as you make sure you use the 32 bit version of the MMC..