Pretty certain there's a guy in this vid riding a softie as a kneelo...he's got it sorted...edit: nah he's ripping the bag out of it...go to 12 mins in...so why the hell not?!
I'll ride anything I can get my mitts on and have a long time, serious lower back condition which recently kept me out of the water for over a year.
In the past I have ridden bodyboards, glassed bellyboards and tried kneeboards. The kneeboards really gave my lower back grief.
Gut sliding waves on surfboards can be good fun since every wave is overhead already but get rolled on a larger wave can be sketchy. Dick Van Straalen rides his longboards prone due to bad hips.
Getting back into surfing again I almost bought a softie but came across this DMS quad fish (Currumbin) that is 6'8" X 23" X 3&1/8" and 56 litres.
Happy paddling and pop ups. Could work great as a big guys kneeboard.