It is just like any other vehicle reimbursement under an Accountable Plan:  You show the proof.

Are you saying an employer can not reimburse an employee's vehicle expenses using the Actual Expenses for the vehicle?  They certainly can.  They just need to show proof of expenses and the business percentage.  Many single-shareholder S-corporations do a similar thing with a Home Office where they reimburse actual expenses based on the business percentage of the home.  The same would apply to an Accountable Plan with an Independent Contractor.