I saw that posting and have meant to pm him also, but hadn't done it yet. Would love to know more details too.
I got mine cheap (like $25?) on Ebay, the seller stated it was for an older model Ironhead. The shape looked similar, so I figured I could adapt it. Well, the seat pan was way WAY off, so I took both seats apart and put the Mustang cover and guts on my S40 seat pan. Didn't like my job with it, so I took it to a prefessional upholsterer and he did a much better job! He added a little leather/vinyl between the seat sections. Only charged me $40 or $50, so I was still happy. It was missing a stud or two, but I ordered new studs directly from Mustang, so that worked out too. I'm glad I did it, it is very comfortable and I love the looks of it. (the slight hump is a raw gel pad, which he said would smooth in, but it hasn't yet, so I'm gonna take it apart and adjust soon) Easy swap? No. But worth it to me!
I don't ride a passenger, so I can't help with that part, sorry. I have sat on it and it seems like it would be pretty comfortable. I'll get my son to take me for a ride on it someday, so I can check out my own passenger seat!