Why couldn't there be a DRK/BTC decentralized exchange? Use the masternodes to host the portal of buys and sells (when an offer is agreed on, the ad is removed from the portal), when someone wants to buy/sell, each respective party puts up their side of the coins to the masternode network, once both send their end of the deal, the masternodes exchange to the other party. In the event one party backs out, the masternode sends it back to the original party. The portal would show up in each client wallet under an exchange tab.
To be truly DRK, you'd need a trustless way for a masternode to hold bitcoin. Maybe masternodes taking part in it could have their server do bitcoin walletty things to receive and send a limited amount of bitcoin, up to a total amount that the bitcoin sellers would be happy for the masternode's 1000DRK to act as collateral...
But they don't need to hold the actual coins in the same way they don't hold actual drk... because that's dangerous.
They shouldn't and don't currently "hold" anything as I understand it.