The problem with post_processing.tbl is that it only includes post processing effects that can be applied through sexps, effects such as bloom which are launcher controlled aren't in there. Which again means that like ambient lighting settings, they vary from one extreme to another. So it is unlikely that post_processing.tbl can ever be tweaked so that it satisfies vast majority of people as defaults, especially if you can't override these settings without table editing or sexps.
Personally I would love if it was possible to control how a mod looks and feels like, these including bloom and ambient lighting settings. Alas, it's not possible to force any settings or even set defaults for settings that aren't in post_processing.tbl. It has long been said that a mod settings tbl file or something similar should address this problem, but nobody has really began to tackle the issue yet. But I suspect that kind of setup would not only require a new tbl, but also redesign of how launcher works, because there has to be settings that user cannot change and settings that are set as default but user can change.
Even so, I doubt mediavps could ever force any settings and setting defaults would only result in more or less people moaning and whining. When it comes to mediavps, you can hardly ever win.