I've been working on a program that does something like this. I've altered the icons on my Mag 315 to geocaching type icons. Also, I wrote a program to change the symbols in .loc files to match these icons. Have a look see at http://home.mn.rr.com/pbe/
Though the 315 hack won't be useful to other model owners, the HMTL conversion program might. If they really don't have any use for day markers, buoys, etc, they may be able to get used to how it looks on their gps.
Right now it only changes geocaches it finds in HTML, but with a little gentle coaxing I could make it work as requested above