I'm aware that both GitLab and Jenkins have integration points with one another, however for reasons beyond my control I am not able to use either.
It's easy to pass parameters to a job telling it which branch, even which commit to build. However I just can't seem to tweak it to the right configuration where it will build the merge request number I pass in as a parameter.
I need to do this with the out-of-the-box 'git' functionality in Jenkins. (Can't use the GitLab Merge Request plugin because it requires polling of the repo.) This job must be initiated manually, and the merge request number specified via parameter. I will not be triggering it with a webhook from GitLab either. This requirement is a manual and on-demand build of a specific merge request.
Is it possible, and I'm just missing something (not) obvious?

