TIRED OF THE JERKS ON THE HIGHWAYS IN USA THESE DAYS?
You don't want to mount a PC monitor on your bumper. But your back window could take a small EDDIE BAUER, AUDIOVOX, or UNIDEN TV monitor. All you need is a A/V jack on your laptop or tablet and a 9-12V hookup for the monitor (or a VGA to AV convertor). The LCD monitor has it's own back-light and you could display large characters in a HTML marquee (times square scroller) or big images the driver behind you can see easily. Even when his high-beams are in your rear view mirror. blinding you
The police probably won't stop you just as long as you don't FLASH it off and on constantly, or use it and be distracted from your safely driving, or using road rage inspiring colorful epithets to piss-off truck drivers (or lorie drivers in UK). Use it for polite humor, or your "tame" political views. Don't use it to get into a fight with some jerk please. Use the smallest video monitor you can find. Not so tiny that the driver behind you can't see it.
You can also rig up a speech to text program and just put your words on it. However, don't use it while angry! :-)
Always mount it BEHIND the driver (you) so as not to block your rear view. Use preset menu buttons or drop-down menu so you are not distracted while driving. Each menu selection sends a different canned message without you driving and typing. Always remember that it's a small screen in rear window so the image and text will have to be BIG.
Run all cables under carpets and seats so as not to get passengers feet entangled in them. Secure the monitor so it does not fall over on turns and inertia. Hide it when you get out so bad guys don't break-in to steal laptop AND the video monitor. Put a black cloth over it or something.
The best message is SUNDAY DRIVER GO AROUND Watch the muscle cars and Harley's ROAR around you in disgust with that one! They may flip you off and call you some choice names while laying on the horn. Being PC is just not "in" today in USA.
P.S. - Using a small VGA monitor is too problematic as they usually don't all have 12 VDC jacks, are way too big for your rear car window, and are difficult to electronically split off the laptop or run long cable runs to rear window. Video monitors are low resolution but will work well with LARGE letters and images at a distance. This is no different than mounting a rear seat mounted DVD player for the kids. Except it's further back in the rear window facing other drivers behind you. It won't work well in direct sunlight though. Only at night is best.
To use a E-INK display to overcome bright lights you will need an old NOOK SIMPLE turned sideways and linked to your laptop via a portable WIFI hot spot. It can remain on batteries or mini-usb car lighter power supply. You'd just set the NOOK browser to your local web server running on your local car hot spot. Have your web pag reload every minute to update any changes you made to it. However, there is no way to make the NOOK SIMPLE refresh it's browser screen automatically. It has to be done manually.