Could you possibly share your expertise (as per my other comment) about why mouse clicking seems to work on PMDG 777 despite I believe this uses WASM and not html/javascript?
Also with your idea about passing x/y coordinates into the wasm, I propose another solution to @iniSteven which may significantly reduce those simconnect overheads. Why not have clickable regions - i.e. pass the corners of 2D polygons as x/y coordinates through simconnect as "clickable regions", each representing a button or field on the current MFD page which can be clicked, along with an identifier. Then have an LVAR which can trigger one of these regions to be clicked.
And/or another solution would be to pass the currently displayed mfd page as an LVAR string with some markup language style tags, with clickable buttons/fields having a tag associated with the LVAR value which triggers it.
Then an external dev could produce the basic graphics to reproduce the MFD on their home cockpit or software add-on.