highwayman - скорее грабитель с большой дороги, чем взяточник.
rogue теоретически может быть с плюсом (vigilante)
impersonator/phony - не сотрудник полиции.

“Rogue traffic cop” – не метафора и, к тому же, слишком общо – нет акцента на мздоимство..

Разумеется, и не скорее, а вообще. “Highwayman in uniform/with a badge” же – метафора, уподобляющая гаишника-взяточника грабителю с большой дороги – то, что и требуется аскеру (или вариант того). Гуглится, между прочим..

just for the record (~17:00):

да, конечно не impersonator/phony officer.
не на тот вопрос ответил :))

[off-duty or not] unfair police officer (traffic cop, HP officer) who violates your right and extorts money from drivers to overlook a citation (protocol?).

Interestingly enough, I have the same feeling about your post.. This is from The Economist:

“In the long run, decent wages for the police and other officials should help cut corruption, which itself increases costs. Businessmen, for instance, grumble about the bribes their lorry-drivers have to pay to highwaymen in uniform.”
http://www.economist.com/node/10608636

A couple of more links for doubting Thomases:

“Along the way, we traveled miles of highway through the jungle, and through many small villages, all celebrating the Virgin of Guadeloupe, a huge event here in Central America. We didn't escape without a "speeding ticket," even though Bob wasn't speeding. Pulled over by uniformed National police carrying sub machine guns, we were told we had to pay 1000 pesos (almost $90) on the spot. After much discussion with the officer, and attempts to lower the penalty, we came up with a US $50 and 400 pesos, and went on our way. We understand this is a situation to be expected; we gringo tourists stand out like a sore thumb and the Mexican Federals are Highwaymen in uniform. Hope their kids enjoy a good Christmas.”
http://www.seatryst.us/Nov-Dec13.2010.htm

Having laid the above foundation based on actual *case law,* let me now tell you of the personal experience of a U.S. citizen and the "highwaymen in uniform" who prowl the roadways of this country in search of "marks" from which they may extort monies under the "color of office." What you are about to read is an actual event as it occured in the early morning hours of June 5th, 2001.
http://www.arkenterprises.com/dialch23.htm

Who is silly and contrived now?

Meateater (AmE) Corrupt policeman who actively solicits bribes, but also tries to get even larger payments.
http://www.eurekaencyclopedia.com/index.php/Category:NYPD_Malversation

meat eater: a corrupt cop (not exclusively mobspeak)
http://www.uta.fi/FAST/GC/mobspeak.html

grass-eater

“The overwhelming majority of those [police officers] who do take payoffs are
‘grass-eaters,’ who accept gratuities and solicit five- and ten- and twenty-dollar payments from contractors, tow-truck operators, gamblers, and the like, but do not aggressively pursue corruption payments. ‘Meat-eaters,’ probably only a small percentage of the force, spend a good deal of their working hours aggressively seeking out situations they can exploit for financial gain, including gambling, narcotics, and other serious offenses which can yield payments of thousands of dollars.”

Впочем, the word "GAIshnik" уже метафоричен: англоязычные и всякие экспаты и так считают - a normal GAIshnik is usually corrupt and solicit bribes. And if you don't believe me, google it yourself.

Now I'm puzzling my brains over choosing the best variant...)

freelancer rogue cop - I think this variant reflects the very nature of the phenomenon.

Still I prefer using a footnote. Freelancing Gaishnik would work too... I also had this thought - wouldn't a rogue Gaishnik be one who doesn't take bribes?!

P.S. Tamerlane, прочитал Ваше "franc-tireur" и вспомнил, что у нас действительно были
такие гаишники-партизаны. Одевались ребята в какую-то идиотскую форму, называли себя типа "экологический патруль" и стояли (в основном) на дорогах, ведущих в Крым. С полосатым "жезлом", а как же без него. Многие, особенно российские автотуристы, велись на разводку. Сейчас вроде прикрыли лавочку, but you never know for sure...Легенда, во всяком случае, пока жива.

аскеру: Just drop the word variant from your vocabulary. Completely. Forget it exists. Use option instead.