I understand that devices supporting ePub 3.0 accept SVG as image format. I however wonder is there is a way to add a fallback image (say, in JPG or PNG) so that an older device may choose to display the latter if it does not support SVG.
Great question. I checked the Kindle Publishing Guidelines and there's no instruction about that. (CSS Media queries can assist in some cases though).
Here's a page about SVG fallbacks for the web browser. I suspect that at least one of these tricks would work on Google Play Books, unsure.
The third option is to use java script to test and swap out images. Kindle doesn't really support that though.
Frankly, I would spend more time testing to see if an SVG is supported on a representative number of reading systems within one platform. Ever since we have had KF8, some of the platforms have improved the way they render something on different reading systems within that platform.
Your approach is wise, and I suspect that documenting fallbacks will be an important first step to implementing svg. But we don't seem to be there yet on Kindle. Maybe not even on iBooks as well.