Default RLI field based on parent Opportunity

Hi all,

I have a field call Branch that exists in both the Opportunity and Revenue Line Item modules.  When an RLI record is created, I want the Branch field in that record to be set to the same value as in the parent Opportunity record, unless the user has explicitly set a value.  I thought I may be able to use a Process Definition for this but it doesn't appear to be possible.

Any suggestions much appreciated.

Dave Hodgman