I think this is an excellent idea! The Global Open Data Index is the most logical place for this assessment and it would be valuable having open data advocates vetting SDG-related open data progress.
Great idea! Or more specifically data availability to measure the SDGs could be tracked. At the German chapter of OK(F) we recently launched an alpha version of a tool to monitor the implemenation of the SDGs in Germany accessible at www.2030-watch.de. The tool is to be continouisly improved through feedback and collaboration. Therefore the code is openly available on Github. Currently we have 55 indicators online with data that is readily available. But for some very important indicators there is currently no data available so we are planning to point out these datasets as well.