Based on some tests on https://git.stg.centos.org (and git.dev.centos.org) and some discussions with @pingou (thanks !) , we found out that the "collaborator" acl actually doesn't let you merge code, even if technically you have access to the target branches.

It seems current pagure code is looking only if you have 'commit' right , instead of verifying also if you have 'collaborator' right, when analyzing the PR for merge operation :

https://pagure.io/pagure/blob/master/f/pagure/ui/fork.py#_348

Also, based on this , we had a look at the pagure api to see if that was easily possible to add 'collaborator' right through API calls, and it's not listed (and not present in code it seems)
https://pagure.io/api/0/#projects-tab => "Modify ACLS on a project" : lists 'ticket, commit, admin' but not 'collaborator'

So worth looking at both at the same time ?