Unfortunately, I am not familiar with Adobe Flash HTML5 Canvas documents but it might help to think of a Juicebox gallery as a complete self-contained web site. A Juicebox gallery is essentially a web page ('index.html') with many resource files (CSS, JavaScript, images, fonts, etc.).
If you can load a web page (or a small self-contained web site) into an Adobe Flash HTML5 Canvas document, then you should hopefully be able to load a Juicebox gallery (via the gallery's 'index.html' page).
This would be more like loading a gallery's 'index.html' page into an iframe than actually embedding the gallery into an existing web page by following the regular embedding instructions here. (The gallery is actually embedded into the 'index.html' page with the required embedding code. There is just nothing else on the page.)
I'm not sure if this will help (or if what you are trying to do is even possible) but it might at least point you in the right direction.