My democrew (Ninjadev) has used this technique for multiple WebGL/Javascript productions over the last few years now.
You can see the final packed .PNG results here: Crankwork Steamfist https://stianj.com/crankwork-steamfist/, Everything is Fashion https://stianj.com/fashion/, and Inakuwa Oasis http://arkt.is/inakuwa-oasis/.
The tool used for creating both the demos and the packed .PNG is made by us and available on GitHub here https://github.com/ninjadev/nin/.