OM: Custom Order Hold Functionality

The requirement is to automatically place an order on hold (prevent booking) if the order total is zero for a particular customer.

It will need to work for manually placed orders or orders from other sources (eg interface).  We are on 11i.10.

As there are no hold source attributes for zero dollar, we need to do a customization.

I thought about trying to add to the hold sources functionality, but not sure if this is the best place to do it.

How does the hold source functionality technically work?  Is it technically possible to extend it?  I noticed in R12.1 and 12.2 release notes they have