Any format is fair game. The more mainstream, used, and simple the format the better the chance of someone taking the time to implement it. I would open a bug at
https://bugs.launchpad.net/calibre requesting it. Put all the information you have found about the format. Such as that link to it's specification and a link to libzim.
I'll warn you that opening the bug will result in it being closed as won't fix. All enhancements which no one wants to work on are closed as won't fix. However, opening the bug shows interest in the feature and allows for someone who might be interested in working on this finding the bug and having the information about the format available.