Each CIP, if you look at this one for instance:
… has a link to a discussion thread. In this case, you can see the Comments-URI: links back to this page. So in theory you should be able to make supportive comments here and they can be used as evidence of a CIP’s priority to the community.
However, based on my experience with another CIP supporting stake decentralisation, there’s no assurance that the developers at the Cardano agencies take forum postings into account at all, nor even read these threads (they definitely don’t respond unless it’s about something they’ve already decided to work on).
You can follow the corresponding developer discussions by finding the “pull request” for the CIP, in which the CIP editors consider merging the proposal into their official list: for instance, this PR link already posted above.
PR comments are supposed to be between participating developers, but since there’s no confirmation they read the forums you can back the proposals there. Just keep in mind we need to keep the CIP discussions on GitHub clear of noise, with comments concise and related to development issues or on the priority of the proposed Rationale and the needs of the community.