There could millions of acceptable issue keys, but using some heuristic we could narrow down and give some probable hints.
End-user suggests the followings
Some equristic may be used in order to narrow down the scope of issues:
1) If a pattern like this "Fixes for <Jira project key>-?" or similar is found, then open issues in scope of this project are subject of interest.
1.1) If project key is found, then limit number of issues relevant to given Jira project to some meaningful number like 10.
1.2) Optionally further limits may be applied by restricting to unassigned issues and issues assigned to current user only.
1.3) URL to Jira project specified by key maybe embedded into reject message too.
2) If no key is found in commit message, then return list of Jira project keys.
2.1) URLs for each Jira projects could be maybe included into reject message like this:
BAS and SAN are Jira project keys.