You are absolutely right! I forgot that MOGRT are basicly in sandbox when used in Premire Pro. I spent hours trying every expression possible, without any good results. To prove your point I searched for filepath info in .aegraphics file after linking new media in Premire Pro essential graphics panel. It really looks like all info is stored in .prproj, and AE simply don't know such info.
So is there really no way to access any of the properties of the replacement media with expressions? This is super annoying since I was hoping to use this feature in templates that need to be responsive to things like the footage aspect ratio.