I think it's possible to use Automated Actions, but not quite in the way I originally expected.

The full details are here: Use Automated Actions to set income and expense accounts

This is the Python code

for record in records: 
    record['property_account_income_id'] = record.categ_id.property_account_income_categ_id 
    record['property_account_expense_id'] = record.categ_id.property_account_expense_categ_id

Hi Lars:

These fields don't have defaults defined out of the box. Someone may have defined default values for the accounts. 

Activate developer mode. Go to ​Settings > Technical > Actions > User Defined Defaults  and check if there are defaults defined for these fields.