SOAP sounds a little heavyweight to be put into the kernel itself. You must remember that everything inside the kernel is always loaded and that takes memory away from the player. The Mk1 only has 8MB, the Mk2 has 12MB and the Mk2a has 16MB. It's already pretty tight in there if you want to keep a fairly large memory buffer for the player.
There is nothing stopping you from writing a userspace webserver with SOAP support however. You would get exactly the same amount of functionality as if it was built into the kernel. There is code out there that parses the player database and a few fragments that decode the current play order so it's not impossible to do.

- Trevor