Currently there are settings in site.siteinfo.xml and site.sitesetup.xml that control whether a particular site type will be displayed in the site browser or not, and whether or not site maintainers have a choice in the matter.
Attached is a patch that allows you to override the settings in these xml files with sakai.properties, similar to how the siteTypes setting from these files can be overridden with the sakai property "site.types".
The new properties are:
Note that site.types.defaultType is included in this patch, but I'm not entirely sure what it is for. I thought initially that it would be used as the default site type selection in the UI when creating a site, but that doesn't seem to be the case as far as I can tell.