checksum would be MD5.
File structure would be css_[md5]_[md5].css
First md5 is the bundle, second md5 is the md5 of all the files checksum (md5s).
Rather than generating the same bundle for 10 multisites let one generate it and the rest use it. This is assuming that the 10 multisites will use relatively the same css/js files in the aggregate. It's an interesting idea, not sure how good of one it is.