I basically have
three options.
1. fix my SVG example that uses text, it blows up in Kobo's RMSDK.
2. find a way to scale a pure-SVG (text becomes paths) with the current line height. I dislike this solution, because it requires magic to become accessible again. It is a solution that works though.
3. accept square boxes, and hope for a better future. (at least on Kobo, on Kindle it does not work at all)