In theory you could use any asset ON THE BLOCKCHAIN to back any other asset's (shorts)
as an example, use bitGLD as collateral for bitUSD ..
however this would lead to people shorting BTSX against USD (not the other way round) and increase the supply of BTSX for 2B+ ..
and that would break the promise to the shareholders and thus is not considered in BTSX in the near future ..
however, in a mumble session, BM mentioned that one could create a new DAC BTSY to give it a shot