If you use PartitionMagic to set your partitions up it will move XP about for you if needed.
You are correct about BootMagic running from Win98 as it needs to be installed on a FAT partition in order to work, having said that if your XP install was an upgrade it may be using FAT anyway.
I have successfully installed XP with the NTFS file system then used P`Magic 8.0 to create two FAT32 partitions for 98 & ME with B`magic installed to ME. The program suggests the best place to create the partitions then reboots into DOS does the biz and walks you thru the new OS install.
A word of caution tho, I did this with a spare hard drive so it did`nt matter if it worked or not I had no files to lose! Any changes like this could lead to you having to reinstall the whole lot all over again! If this does`nt bother you then have fun.