It is a bit confusing when people mention fixed gear and coaster brake in the same sentence. A true fixed gear doesn't allow for any freewheeling at all and has constant engagement of the drivetrain. I looked up the Harper bike you have and they kind of say the same thing but obviously if they use a coaster brake it has a freewheeling hub if it will coast....Granted it is what has become to be known as a fixie style bike but not a true fixed gear.
Other than the front hub/PAS option, so as not to need a throttle on the bars, as suggested with an external controller the only other option I can think of is the TSDZ2 coaster brake mid drive system as seen here:
Brand Name: Ebird Voltage: 36V/48v Design: Brushless Wattage: 201 - 300w Motor Type: Side Hung Motor Name: TSDZ2 Engine Ebike Kit Application: Mountain Bike,City Bicycle,Snow Bike,Sand Bike Packing Weight: 5.2KG LCD Display: VLCD5/XH18/vlcd6 Chain Wheel: 42T Feature: Coaster brake mid drive motor
recycles-ebike.com
This would do the job but you would need to have at least the minimal display VLCD6 display for the system to function as well as a battery of course that would suit your range requirements and of course a smaller one enabling less range would be the most discreet and if kept close to the motor minimal wire exposure.