It all depends on what external source you are tapping into. We use active directory so we are tied to ldap. We have two domains so I had to duplicate the plugin to access both of them. It is working really well and also allows to use LDAP enrolment into courses and I also assign users to the site by the use of a security group (not in the Moodle group, you can't access the site!)
I imagine that external database might work the same way - I have just never used it.
If someone hasn't come up with a patch, the plugin duplication takes
some work as you have to go through the files and change the name
wherever it is referenced.
The good thing is that once you have the duplicated plugin set up, you should be able to share that with your other sites for fairly easy install or just run the patch that Inaki has so kindly provided for us to use. Just note that it will need to be addressed again on any upgrade...