Does it make sense towing airplanes to the head of airstrip by external (eg electric) means?
Was [towing unit or motors in landing gear] ever tested or assessed?

Yes.
2014 Main Gear

Using the Auxiliary Power Unit (APU) generator to power motors on the main wheels, the EGTS taxiing system allows aircraft to push back without a tug and then taxi without requiring the use of the main aircraft engines. One wheel on each main gear is equipped with an electric motor to drive the aircraft

From EGTS

Airbus signs MoU with Honeywell and Safran to develop electric taxiing solution for the A320 Family

From Airbus press release

Lufthansa Technik has proven with a technology demonstrator that aircraft can be driven on ground by electric motors.

From Lufthansa Technik (with video)

2015 Towing Unit

As of 2015 Taxibot, a semi-robotic towbarless tractor which meets and connects to aircraft, is the only alternative taxiing system certified and currently in use by airlines in the market.

From Wikipedia

2016 Nose Gear

Honeywell and Safran have pulled the plug on their jointly conceived electric taxiing system, while rival developer WheelTug aims to enter service with its system in 2018.
&quot;After thorough market analysis&quot;, the US supplier says, the two partners &quot;agreed to stop work on the Electric Green Taxiing System due to dramatically lower oil prices and the current aviation industry's economic environment&quot;.
EGTS had been scheduled to enter service this year, with Honeywell and Safran having planned to produce a system for line- and retrofit on A320-family and Boeing 737 jets.
Meanwhile, Gibraltar-headquartered WheelTug plans to enter service with its nose gear-mounted electric drive system in 2018.
... initially on 737s and, at a later stage, on A320-family ...
Neither Airbus nor Boeing are supporting the programme

From FlightGlobal