I rode rigid MTB's aggresively offroad till 200. Reason for front suspension was that I needed it after a bypass and I could not take the knocks the chest was taking.

But onroad- there are other ways of getting a "Softer" ride. Unless you are talking $$$'s then front suspension will not be good. It will take the sting out of the bumps a bit but Pogo Stick comes to mind- and weight- and loss of propulsion.

Just by using a wider tyre with less pressure in it will give a lot of comfort without a great loss of energy. Better quality wheels with a lacing pattern on the spokes that has them crossing 3 times will assist a lot. And hate to say it- improving your riding style to make the arms and legs act as shock absorbers helps even more.

Suspension seat posts are a problem. I have an offroad Tandem and suspension for the stoker is a necessity. Cheap ones caused problems and broke freqently. I had to spend a lot of money on a Cane Creek "Thud Buster" to get one that worked and did not cause problems. So if you want a comfortable ride- get a top quality suspension post and use the arms to take the bumps. Or as I have said-get better wheels- wider tyres and modify the way you ride to make it comfortable.