It really shouldn't be too hard to implement, at least on the Steem side. Here's how it would work:

Three parties involved:

A has Steem to exchange.
B has fiat to exchange.
C is an arbitrator, needed ONLY in the case of a problem.

A and B agree on an exchange of a specified amount of Steem and fiat.

A deposits Steem to an automatic/programmatic interface along with B's designated receiving address. A cannot remove the Steem, but can enable it's release to B.

When B transmits the agreed amount of fiat to A, A then releases the Steem to B.

There are two failure cases, requiring the involvement of C.

First case is that B does not transmit fiat to A.
A then goes to C to seek the return of his Steem.

Second case, A does not release the Steem to B.
B then goes to C to seek release of the Steem.

The ideal setup would be a fully automated system that could see and control both transactions. "Mailboxes" if you will, for both Steem and fiat, headed in opposite directions. The system would note when both mailboxes were full of the agreed amount, and would then release both to the exchangers.

If both mailboxes were not properly filled within some specified time window, whatever was put there would be returned to sender...