It can be done using Automated Actions.  For example, to default Routes on stockable products:

• Model = Product Template
• Action To Do = Execute Python Code
• Trigger Condition = On Creation
• Apply On:
  • Use domain to select which products are updated. In this example, we are only only updating stockable / storable products, though you might want to include consumables as well.

Python Code

In this example, we are selecting Buy (5) and Make to Order (1). Multiple routes are selected by entering the Records IDs separated by commas.

record['route_ids'] = [5,1]