The ImageMap extension emits markup with typeof="mw:Extension/imagemap", not standard parsoid image markup. For this reason, the bundler does not include those images in the bundler (and the renderer does not render them, either). The GraphViz extension is also affected, since it uses ImageMap for its output.
Note that GraphViz emits markup with typeof="mw:Extension/graphviz" and typeof="mw:Extension/mscgen". So, even though GraphViz uses ImageMap for generating HTML output, making the OCG bundler and renderer recognize mw:Extension/imagemap won't help GraphViz.
(because to be honest I haven't even been able to get electron working locally yet)
Oh dear, who would have guessed that mwlib is still the "easiest" "working" solution even years after it was abandoned. I am a bit scared now. O_o Still there is no way around going electron I believe so if you could document your findings on how to do it once you have succeeded doing it that will be great. Very much appreciated. Keeping fingers crossed.