Sorry, webber, that's a negative.
A few things to consider:
- Synthing means comparing each photo to every other photo. Since finished synths are online and new photos are on you computer, this is problematic. It also means that every new photo would need to be compared to every existing photo anyway, taking just as long, and every new photo potentially changing the positions of any of the other photos in the 'Reconstructing Scene' phase of synthing - the most computationally intensive and time consuming part.
- No photo is ever uploaded to Photosynth servers twice, unless edited between uploads.