With a small change to bitcoin protocol it should be possible in some cases. But right now the only viable option is to use M of N oracles:

https://github.com/orisi/wiki/wiki/Orisi-White-Paper http://gavintech.blogspot.com/2014/06/bit-thereum.html

Basically, you choose a panel of up to 15 independent entities to verify that the funds were locked indeed. The panel can be modified by a majority of votes if some of the entities prove to be unreliable - kind of like the Supreme Court works.

This obviously introduces a new vulnerability to the system, but is the best solution available right now.