XP x64 edition was based on the same code as 2003 SP1, which is why it ships with IIS 6.0 instead of 5.1.
So, if you have a tool that can manage IIS 6.0 (as you mentioned with RSAT), it should run against the XP installation. Doesn't matter that it was designed for 2003 server, functionally they should look (near) identical.
The other possibility - which I almost hate myself for mentioning, but I've started now - is that the IIS 6 Management Console included with IIS 7 might be usable remotely.
What is for certain is that the IIS Manager for IIS 7 in Windows 7 will not manage an IIS 6 installation.