Include only requires the src attribute, nothing else. I'm not sure what else you're thinking it needs?
As far as the directory goes, you can easily specify an absolute filename. The caveat is simply that it's rooted at [acs_root_dir] (e.g., /web/myserver/), so you'd typically want to say something like /packages/my-package/www/my-include, or /packages/my-package/www/lib/my-include, or even /packages/my-package/inlcude/my-include.
This works just fine, and I actually think it's quite desirable, as you don't want to allow people to source stuff from random places on your server's disk drive.
