Well, admittedly my first experiments’ results proved less than wonderful. As you can see, I went with a QR shape made of card board, diligently hand-cut from the Amazon package the chalk spray came in.
While the first tests using white paper instead of chalk colour under the Matrix shape turned out quite promising, the final result on asphalt was not readable at all, despite having added the mandatory "quiet zone" around the actual code display (not shown on the picture) before running the readability tests.
Anyway, with future expriments I’ll start with a more even surface as a background and less colour, so that the individual squares hopefully will not bleed into each other as much as they did with this first print.
Will keep you updated about future outcomes.