Searching for the commit hash in the “Search for code or repositories…” bar should also return search results whose commit hash matches.
1) I have a commit hash
2) Paste the commit hash into the Bitbucket search box on the top right
3) Get a result to take you to the commit and/or the project the commit hash belongs to.
In the case of collision, just show all matching ones respecting repository access permissions.
Our regression and QA team often times will have to bisect regressions. When doing so the only information available at the time is a hash. Bitbucket should make it easy to find out commit information about this hash without needing a checkout of the codebase.