A highly desirable function is the ability for users / domain owners to install scripts. Should not be limited to virtualmin server owners.

Domain owners can install scripts, though it apparently isn't the default in recent versions (I'll get this fixed in the next release). Click on the domain in the index page, and click the "Edit owner limits" button. Set the option labelled "Can create and edit sub-servers?" to Yes. If you don't actually want the user to be able to create a bunch of sub-servers, you can set the limit to "0". This is also a confusing way to restrict owners--the sub-servers feature enables a whole different view for the owner rather than just enabling sub-server creation. Something about this option and feature will be changed to make this more intuitive in a not distant release--this has been the clear winner in the "What's the most confusing thing about Virtualmin Professional" contest. ;-)

Holler if this doesn't do what you want.