Indeed the low granularity makes the Big Mac price a cute wrapper around a forex pair, to state it bluntly. Forex pairs need leverage to be interesting for traders.
I think we could easily take the median price and thus probably leave Belgium out. Another thing is that I always take a meal as a separate Big Mac is relatively pricey. 
The initial thought was to track the Purchasing Power of USD and not the PP parity to another currency. Although 20% of the total supply was created last year alone, the demand for the USD seems to be ever growing too. Both in forex markets and crypto it is completely dominant. China first needs to establish the Yuan as a reserve currency by becoming as transparent as the Fed.