WP 3.0 menu system strips out "rel" info from links

I’m trying to add rel=”” info to one of my navigation items that I’ve created via the WP 3.0 menu system. I’m using the “rel” to activate a lightbox effect. The lightbox script also uses “rel” to display the video in a specific style.

This is what I need my “rel” to be:

shadowbox;width=500;height=306

But the WP menu is stripping the html character identities and inserting the info as this:

`shadowboxwidth500height306′

I’ve tried making it with the character identities:

; is :
= is =

Does anyone know how to pass this info through to the ‘rel’?