Great thanks - perhaps obvious, but worth running the PNGs through something like pngcrush.
I'm thinking about it the other way also, that is could your approach be used to reduce the size of DEMs encoded as PNGs. While I can see brotli being more efficient, by not using a image compression algorithm you perhaps lose out on exploiting the 2D nature of the data, as if I understood correctly when you're compressing the data you treat the tile as a 1D blob of binary data.
I'm thinking about it the other way also, that is could your approach be used to reduce the size of DEMs encoded as PNGs. While I can see brotli being more efficient, by not using a image compression algorithm you perhaps lose out on exploiting the 2D nature of the data, as if I understood correctly when you're compressing the data you treat the tile as a 1D blob of binary data.