Is there a way to start a script on the publisher when synchronization starts? I am using merge replication with pull subscriptions and Web Sync.

What is the easiest way to start a SQL script on the publication database when a subscriber starts the synchronization process?

There is this, sp_addscriptexec, but that specifies a script to run at the subscription database which isn't what I am after.
There isn't a built-in or easy way to do this with Web Synchronization.  You're best bet is to programmatically monitor Merge Agent sessions at the Publisher/Distributor using sp_replmonitorhelpmergesession and sp_replmonitorhelpmergesessiondetail.  You can script this out and poll on a schedule.