I've seen the XML code listed on another forum... it used to be listed here, I believe, but was found to be the Leisure Linx rate code, so they took it off. I've tried it myself in searches but in the comments it sometimes said ID required, so I have never used it.

On another note, yes, there are several publicly usable rates that aren't listed here; I think it's just because it's hard to find/keep up with every single one. Here's a couple I've found just on the Marriott website that are for various packages (it's just a matter of locating them on the Deals page):
L19, M41 (Members bonus bucks), B18, ROM (Romance), ZZ1 (stay and play), MW1 (point savers), ZH4 (family fun).

*/end lurking/*

*edit*
Oops, looks like someone posted while this was sitting. <_<;;