Basically you're right. If it's not included, it's because no one pushed a change for inclusion. Wireshark includes as many protocols as possible and zookeeper would be definitively accepted.
Before starting, be sure that something doesn't exist out there: you could start for someone else's code (according to its licence, of course) and not start from scratch.
When submitting the dissector, please be sure to add sample captures for testing. To add them, just file a bug on bugzilla with your attachments, and add a reference "Bug: XXX" in your commit message that will automatically link the bugzilla page.
Dario.
