Polychords have been discussed before, but generally the discussion seems to come back to how to create a specific chord by using other means - such as using the slashed chord notation - which simply places the lowest note, or some other methods using copy paste, manual insert etc.
Is it not possible to use a slightly different syntax, such as:
chord1 '|' chord 2
chord1 polysymbol chord 2
where chord1 and chord2 are just the "normal" (possibly complex) representations for chords, while the '|' or polysymbol simply indicates that the chords are to be played together?
polysymbol might be '|' (perhaps the default), but otherwise could be any other symbol or combination which makes visual sense and doesn't conflict with other features. For example it might be ||
I don't know what the status of this kind of issue is now in the latest versions of Musescore.
I don't know enough about the internal workings of Musescore, but using a syntax directed technique to parse chords should work well enough, and would be a lot better than using ad-hoc code and patching in a kludge to suit every new suggestion.