In my opinion, the way to do it would be just a continues flow of sbds to internal market buying all steem available on the ask. As this service is continuous it would be difficult to game, just one continues buying of steem available on the internal market. The internal market is mainly run by bots that are continuously replenishing their supply of steem as buy orders take them out, and this replenishment of steem eventually comes off external exchanges. If things get mispriced on the internal market vs the external markets then users will likely arbitrage the difference to keep the prices close.