Pre-Sync Event Possible?

I have a requirement to run some code before a particular synchronisation event occurs. I can write the process in VB but the only place I can see to execute such a process is through Process Orchestration in Designer and the only options I get is to run the piece of code on an Insert, Update, Delete etc. So is there somewhere that I can hook into one of the synchronisation processes and run some code before it occurs. Another option would be to run some code after the sync but this is less desirable and would result in more complicated code. Can someone point me at where I could implement this kind of thing in the product or some at some documentation?