Overlay a small icon (or not, in the case of microb) at the bottom-right corner of the globe that represents the actual browser for that link. If you're forcing people to reboot after changing the default browser, this should be picked up nicely.
And instead of renaming Icon: entries in .desktop files, for instance, just actually overwrite the actual .png files.
Then you could start having 3 or more browsers by centrally configuring them as: browser (default) Actual-Name: Tear Label-Name: Web browser2 Actual-Name: microb Label-Name: microb browser3 Actual-Name: Midori Label-Name: Midori ...