OK I admit to not fully understanding bitshares so this post could be way off. If so sorry in advance! :/
OR this is crazzzzzy awesome man!! So... I think bitshares can do direct BTC exchanges.
Just like with Price Feeds, delegates can "price feed" a public bitcoin address.
So a BTS:BTC market could be managed trustlessly in the bitshares client like this:
1. The bitshares wallet is updated to include bitcoin wallet features.
2. A standard BTS market is created but BTC bids include the target bitcoin public address as meta-data.
3. Using the bitshares client a BTC holder can accept BTC bids and so the bitshares client broadcasts a bitcoin transaction encoded with their bitshares public address (and maybe the bid ID).
4. Since delegates have a "price feed" mechanism they can monitor the bitcoin public address and after 6 confirmations the delegates can safely process the BTC bidders side of the deal by releasing the BTS tied up by the bid to the bitshares public address encoded in the bitcoin transaction.
5. If the BTC bidder wants to cancel they would need to wait like 2 hours so delegates can make sure no bitcoin transaction is processing.
6. For BTC offers only the BTC holder can trigger the market transaction so BTC holders would need to be logged in to the bitshares client otherwise their offer would be grayed/suspended.
Sounds too good to be true. But I thought I should at least share the idea.