I am sharing a server farm between multiple business units.

I would like to have the following arrangements

1) Some custom site templates available across all virtual servers
2) Some custom site templates available across all site collections within a virtual server
3) Some custom site templates only available within a site collection

3) is dead easy via web interface via top level site admin

1) is acheived by command line

How do I do 2) ?
I cannot see anyway of achieving this and then being able to create top level sites based on a template. I must be missing something ??

We are hosting in a "consolidated" standard manner
http://businessunit1/sites/onesitecollectionname
http://businessunit2/sites/anothersitecollectionname