Hi Andrew,
I'm currently working on a map project for the Santa Cruz County Search and Rescue team and dealing with a similar issue. The people on the ground want UTM coords and the pilots want decimal degrees. My initial thought was to set up the different grids to use and then export each as a graphic, then convert the graphics to a feature to have access to it in the TOC.
Obviously this is not an ideal solution but I'm hoping it works. I think the biggest problem is going to be the loss of dynamic labeling once converted. I'll let you know what happens if I give it a shot.
Best,
Aaron Cole
GIS Analyst, Center for Integrated Spatial Research
University of California, Santa Cruz