Not a bad idea, but I'd think the market for that is too small to warrant its own business. Existing exchanges could implement a gold/btc exchange to reduce the marginal cost of creating the service but then it'd suffer from all the problems that physical exchanges suffer from (security, privacy, the increased friction of requiring customers to travel outside their homes, etc). Gold is also harder to store and transport than cash and being one of the first/few institutions that trade the pair means they'll have to exchange the gold for cash, then for Bitcoin if they wanted to increase their exposure to Bitcoin on their balance sheet.