I have a friend who built an enclosure with an opening through their basement window. They go in and out whenever they want. It has a big tree branch and a bunch of ledges for them to sit on. It's built with 2x4's and chicken wire. That might be your only recourse. They're pretty simple to make. Got a spare basement window??? It's worth it so you don't have to worry about Mr. Macho A-hole killing one of your cats.
The only other option I can think of is indoors only with Feliway Defusers.
Keep us posted.
Gary,
http://img.photobucket.com/albums/v6...Picture675.jpg
This pictures has "GOT MILK" written all over it!!! Too cute!