I would go with the MultiTenant plugin. You say that the views and logic are different, but I assume that it will differ more from the data? Use MultiTenant to cleanly share data for each site without any noise, and if you store the configuration of views and logic in the database, you get this unique value for each site.
I think MultiTenant is upgrading to the latest version of Grails, at least I saw it in the Grails email list. I think you need to use version 1.2 until then.
I am doing something similar to single data using the Hibernate Filter plugin. My views are the same, but I have texts and messages on the site, first looking at the message "SITE-A.hello.world", and if the default message "hello.world" is not found. Just to give you an idea of how you can customize your views, although you probably need to take it further than for your system ...
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