I have installed a regular WP plugin that has had good reports of working friendly with WPMU. I looked on my Supporter Premium Plugins page and made sure the radio button was marked as available to "None."
I logged into my test blog and Lo and Behold, the plugin was showing under settings as pretty as you please. My initial reaction is that Supporter should have caught this as one of the main reasons for it was the ability to offer Premium Plugins to Supporters from plugins added in the /wp-content/plugins folder.
The plugin is Greg's High Performance SEO plugin downloaded from the Wordpress.org repository.
Advice and/or solutions anyone?