There is an officer and a CI on the committee there to represent their respective groups on any process.
Is it proportional in terms of the ratio? No, but it never would be even if all roles are completely open. However, their input carries a lot of weight as they are there with the specific purpose of representing all officer and CI members. The truth of it is that actually the Mess' impact is very low in terms of actually affecting anyone or any group in any meaningful way - the choice of venue for a function or event isn't likely to cause much of a revolution.
It is possible however, that in a completely free committee, these positions would be lost causing a worse skew in representation. If the committee became loaded and one group or more didn't have a member, that would make it less representative than it already is...
An alternative scenario is that the majority group just votes everyone else down through some kind of elitist power grab such as you've described - if, of course, those on the committee were so inclined.
You couldn't nominate specific committee roles for specific ranks - that would still be discriminatory.
Could it be done? Yes, although you need to be careful to avoid some of the problems you've already attributed to Messes like we already have.
Is it worth changing an existing, successful system for the sake of it where no opposition from within seems to exist? Debatable.
Might even ours change in the future if there becomes a desire? Yes. I don't think our current committee are unreasonable folk and who knows - there'll be another AGM soon enough.