Hi!
I tested with the latest JUCE download as of yesterday, and Multi-Touch Long-Press Right-Click doesn’t work. I asked a question here on that last year, but didn’t get to the bottom of how to achieve a good compromise, and had more “pressing” (hehe) things in my task list.
In summary: If I disable multi-touch in JUCE as in this conversation, long-press with a finger on any GUI component that receives right-click events works, for example, cut/copy/paste menu in JUCE’s code editor component - but I lose Multi-Touch. With JUCE unmodified, the long press does nothing.
Meanwhile, I’ve tested on Strobe2, and somehow the JUCE colleagues at FXPansion have figured out what seems to be a functioning solution on this!
Multi-touch works in Strobe2, and so does “long-press right-click”. Can’t you ask them how they did it, and incorporate it into the library? 
Dragging two knobs on Strobe2 works, and so does bringing a contextual menu up. The rectangle under Dos is what appears in windows after a long-press:
