Devise a plan to test tagging a release candidate using a specific commit

Context

As part of gitlab-com/gl-infra/software-delivery&1 (closed) the ongoing stable branch will be generated one week earlier in the release schedule to allow for thorough testing and timely backports.

Implementation details were put in place to automatically create the initial and final release candidate. These were tested on #21327 (closed)

Problem

In some rare scenarios, the initial release candidate can be created using a specific commit. The new implementation supports this case https://gitlab.com/gitlab-org/release/docs/-/blob/master/general/monthly/release-candidates.md#default-creating-a-rc-via-monthly-release-pipeline. However, this was not tested during #21327 (closed). To ensure this functionality is working, it needs to be tested.

The purpose of this issue is to think of a strategy to test the release candidate creation with a specific commit without interfering with day-to-day operations.