Even Granny can short BTC with Ripple.
All it takes is for someone to accept Granny's BTC IOUs in exchange for more liquid BTC IOUs like Bitstamp's.
Granny can then sell the Bitstamp BTC IOUs for USD IOUs, and wait for BTC to crash.
Granny is now short Bitstamp BTC.
Short selling require the seller to borrow the coins. Hence, a counterparty is required, i.e. someone holding the long position. Why should anyone trust Granny's IOU just like that? I can't see how this can be done without a broker, just as in conventional markets. Though I must admit do not understand Ripple completely, so perhaps I missed something here. All it takes is for someone to accept Granny's BTC IOUs in exchange for more liquid BTC IOUs like Bitstamp's.
Granny can then sell the Bitstamp BTC IOUs for USD IOUs, and wait for BTC to crash.
Granny is now short Bitstamp BTC.