I ran across a russian site a while back that is creating Counter Strike on DC using the Half life engine and files I believe.
You can actually download the demo realeases and play. It's pretty fun.
http://www.csdc.dcfan.net.ru/
I belive beta 4 you need to half life for dreamcast already. But, beta 3 is a bootable .cdi you can burn and play on your DC without half life. Just use Discjuggler program: http://www.padus.com/downloads/demo.php to burn the cd image onto a disc.