If signed commits are required across multiple repositories, pull-request merge commits that exist in a new PR will not be allowed to merge due to the hook. For example:

1. Have Repo A create two forks, B and C
2. Merge a PR from B into C
3. Attempt to merge C into A

A will not allow the merge since the merge commit is not signed.

If Bitbucket could sign its own commits, it would allow for this type of workflow where work is being done in forks rather than branches in the same repository.