What you are talking about is possible, but it is a bit tricky.
The symbol file looks like this:
You can place your own version of that file in your 'default tracks' directory and TF will load it instead. So basically you could overwrite a few of the 'known' waypoint symbols, like trailhead and campground with your own images. Then simply use trailhead for user #1, campground for user #2, when you make the waypoints.
Does that make sense?