Coming from a different corner of the music sphere (old-school internet indie artists releasing a lot of video game arrangements loose on their own websites), I can echo that this would be an extremely useful feature to have. As of right now, I often end up using a Recording Series to group a bunch of standalones together, and end up adding it to my library through Picard under a big [non-album tracks] blob, in place of a proper Release or Album title. This is a good example of a case where I did exactly that recently.
So, yeah, definitely echoing the request to be able to use Series entities as a surrogate grouping type in Picard in lieu of a proper Release, but I have no idea how this would work, as a technical specification, when the individual recordings you’re adding aren’t standalone, but do already belong to proper Releases.