I am afraid that there is no elegant solution for that.
So, if we are talking about one computer with multiple users, you would have to create a snapshot of registry when a browser sets itself as a default (there are couple dozen settings get changed) with registry monitoring software (some uninstallers, regmon, etc.).
When you would have this REG file ready you can inject these settings into other users’ registries by standard command
REGEDIT.EXE /R:user importfile.REG
where "user" is location of the ntuser.dat of a particular user.
Or you could inject the ie setting, which checks whether the ie is a default browser or not. But that would be up to user - actually make it default or not.
If you have one computer with couple users on it, I think you would be better off doing it manually for each one.