additional filter for upstreaming

Hello.

I would like to ask how would one go about applying an additional filter for commits in the upstreaming process. Basically i need to make it possible for devs to manually add a git note to a commit, which this new filter would recognize and according to that skip or include said commit.

I would also need to be able to configure the logic of which flag means what, whether directly in source code, or with arguments when running the git-upstream command.

Thanks.