Some taller riders have reported that where their knees touch the plastic of the fairing, the plastic becomes scuffed. Well I'm fairly tall at 6'2" but I haven't had this problem; my leather trousers incorporate knee padding and I haven't noticed any scuffing of the fairing plastic.
However, for those riders who wanted to protect this part of the fairing, the Honda Accessory catalogue used to list a pair of knee pads which could be mounted on the rearward-facing portion of the fairing. They required holes to be drilled.
Unfortunately these knee pads are no longer available. You could of course make your own by purchasing two gel-filled wrist-support mouse pads and attaching these to the fairing with either glue or strips of velcro.
ST1100 Scuff Pads
When the black 1991 ST1100 was released, some customers complained of the fragile finish, so Honda provided these customers with a set of scuff pads. Later this was also listed in the Honda Accessory catalogue.
The set consisted of ten die-cut clear plastic self-adhesive panels:
Fuel filler door cover
Lower edge of false tank just above seat edge
Side panel front (x2)
Side panel rear (x2)
Pannier forward edge (x2)
Pannier top (x2)
Once again, these panels are no longer available. However, if you're handy with a pair of scissors, it is fairly easy to fabricate your own using some self-adhesive carbon fibre sheet: