I see. The rebasing you mention is part the implied workflow I was missing. I see it now. Yes - if the rule is that merge commits to master are not permitted (ie. all commits must be rebased onto master first), then of course one person cannot rebase someone else's signed commits without losing those signatures. Thanks.

Theoretically one could devise a tool which allows each contributor to re-sign (in the correct order), but I'm not aware that any such thing exists, and it'd probably be too impractical anyway.