Set "Load remote content in messages" per sender?
The only (ugly) solution to date seems to be:
tell application &quot;System Events&quot;
    click at {x, y}
end tell

— where x and y are coordinates of the mouse over the button to simulate a click (You can easily find these coordinates by doing a screenshot, i.e. cmd+shift+4)
Note: This will only work for the first message in a conversation since the button will be at the right place.
Tip: Although a dirty workaround, I accept this compromise as I make it more user-friendly by launching the script via a three-finger click through BTT.