But since you are already using canvas for your game, can’t you instead load up your desired mockup image using the image loading capabilities of canvas as part of your initialization? That way its all inclusive and you have full manipulative control. You can also render mockups entirely with CSS, doing this would again give the ability for further manipulative control.
Regarding the entry points you will need for implementing your source structure into Blocs have a look at the following documentation.
With those links you should be able to manually implement all of what you need through various approaches. Hopefully that offers you some starting points to see how things would be possible with Blocs.