That's true. Since the plugin still relies on them, I kept them in the above example, but they're not required at all.
In my tests, the analogous domain_mapping_themes_url() function (which doesn't rely on constants) also seems to work for plugins, both regular and MU. It does however use the siteurl option to fetch the base URL for replacement --- this may break the plugin in installs where WordPress lives in its own directory, in which case home would be a more appropriate option to use for all replacements.