Cross-signing commits

["Soni L." <fakedme+git@xxxxxxxxx>]

We have a somewhat unusual use-case where we need to cross-sign commits. 
Is there any way to do this in git? As far as one can tell, attempting 
to cross-sign a commit would cause its hash to change, and creating a 
signed child commit would break fast-forward merges. So these are a no-go.

Thanks.