I've worked 80m /M a couple of times checking into MARS on USB about 20,000 over Chicago. Just happened to be in an old Citation with the old Collins USB radio.
I'm planning to put an HF transceiver in my E-LSA Flight Design CTSW. It's carbon fiber so the antenna and ground plane is an issue. Carbon fiber is quite lossy. My top speed is about 120 kph.
I've toyed with the idea of a trailing wire antenna so I could work 80m. I've looked at all the WWII scout planes with the trailing wire setup. A bit complicated, maybe. As a glider tow pilot, I have some appreciation for the foibles of a piece of rope hanging behind a landing airplane.
A more practical idea might be a V from both wing tips to the top of the vertical stab. A tuner in the tail. It will handle up to 20m. Both are draggy, as was mentioned.
Here's the radio I'm hoping will work. CommAir transceiver.
http://www.universal-radio.com/catalog/hamhf/6010.html
I have a PS Engineering 8000C and an adapter to handle the mismatch of microphone impedance. The 8000C handles 3 coms, so I'll be using my regular headset. It's often used by agencies that need to use a non-aviation radio.
(I bought 2 by mistake, so I have an unused PS8000C for sale).
My setup is in the incipient install phase. I bought all the gear except the HF radio which is pending FCC approval. I hope to start the install right after Labor Day. I've received considerable help from local Rockwell Collins aviation radio engineers and from Stein Bruch of Steinair.
I make some long trips in the CT and thought that it might be fun to pass the time by making a few contacts. I agree it is essentially impractical. That, of course, is neither here nor there and I find no need to apologize for or defend it.
Jim NO0B