As it is not possible to change the project identifier in Redmine, it is impossible to change the repo location and still use Redmine.pm. I believe a simple patch could be created that determines the project identifier based on url. I.E the following database query.

SELECT project_id from repositories where url like '%url%';

The patch should take into account that like may return multiple projects IDs and should fail in this case (This should be made clear in the documentation). An option, RedmineByURL could be added to enable this feature.