A couple of questions which I think were touched on a little:

First, would the recommended approach be to add calls to the API?  For example, a call that allows low-level crafting of the transaction or at least the script could be used by multiple UI's so that the presentation is separated from the inner workings.

Second, the case of buyer-seller-mediator can be handled with a 2-of-3 signature transaction.  My question is: does the mediator get paid in a separate transaction?  What if the mediator expects a small fee if the transaction goes properly, and a larger one if he has to intervene?