I would sometimes like to transport my tandem by car, and it would be convenient to use one of those "strap-on" bumper carriers. Is it feasible to use this type of carrier?
If it sticks out from the with of the car DO NOT carry the bike in the bumper carrier, if necessary, take both wheels out but take no chances, anything wider than the car is a danger to yourself and others, think about motorbikes.
Yes, yes it is possible...
If the tandem is loaded onto the carrier at an angle, it won't extend much beyond the width of the car or minivan (maybe not at all for a larger vehicle). It's not totally clear in the following picture, but the horizontal bars are tilted up a bit to make it more secure.
It's necessary to bungee-cord the bike or otherwise secure it otherwise it'll tend to slide "down" (in my case, the front will tip down, which will make it stick out on the driver's side). I use 2 cords to secure it:
Although it looks ridiculous, the same technique can be used on a small car:
The bike doesn't actually stick out beyond the width of the car as much as it appears to in the picture, it's less than a foot on each side. It does stick way way up above the car, however, so when I've done this (only a couple times) I'm very careful about going into underground parking.