Last I looked, several browser were using something called xdg-settings which updates a file in a hidden dir on disk. What we would need is something like what we did for xdg-open: implement an xdg-settings that snaps have access to by default that talks to an out of process helper (eg dbus service), which is in a position to mediate the access (eg, throw up a confirmation dialog and do the operation on the snap’s behalf). Other applications edit ~/.config/mimeapps.list directly, so they would need to be modified to use xdg-settings.
In the meantime, it might be worth having documentation that says how to set the default browser outside of the snap.
UPDATE: I said “eg dbus”, but I meant the snapd session daemon that works with xdg-open (which is not dbus)