I know that this won't be what you're asking for, but once you move the camera in the desired direction (via cursor key, touch screen, mouse drag, etc.) then press 'Play' then the synth will play in the direction last moved. It can be as simple as a single tap of a cursor key to reverse direction once the synth loads and a tap of the spacebar to resume playback.
I would still prefer being able to specify this myself as the synth author (or potentially the person embedding the synth or including it in a synth playlst) but at least there is a way to achieve this as a user who is simply viewing the synth.
One simple way that the Photosynth team could achieve customizing the synth playback direction as the synth author would be to save the direction which the camera was last moved in when the synth author changes the synth's starting photo.
Having personal preferences for one's account isn't a bad idea, but the Photosynth team have constantly rejected that approach.