Okay, I had a bit more time to look at this today.
The way it looks, the menu options on the VCS menu all point to the VCS^VCSCMDS macro. This macro contains a switch with the various commands (put/get/lock/etc). Each case in this switch eventually ends up calling pkg + command; for example, CVSLock, SVNGet, whatever…
Somewhere in the tool-specific macro file, the command lines are parsed out of the global configuration string, and executed.
Unfortunately, there’s no generic support for ‘just run my command-line if you don’t know what else to do with it’ sort of things.
So, the way it looks, you can take something like CVS.S and copy it to create a new BK.S (don’t forget CVS.SH to BK.SH) and modify as necessary. This is the first time I’ve even heard of BitKeeper, so I’m not sure how big of a deal this will be. I know the Subversion (SVN) file in 9.10 is nearly identical after a s/CVS/SVN/ has been applied.