I'm sure this is well known already, but it was a new one on me.
The thing I learned (was told by an exasperated web designer) is that you can set the "tabindex" of a field to -1. If you do this then it is effectively excluded from the tab order of the form as you use the keyboard to navigate around.
Cue much punching of the sky as we knock another bug off the list for the application I'm working on at the moment. It's the little victories that count!