If you know someone who works in fibregalss, have them reinforce the inside of the seat unit with kevlar sheeting or similar, that way you'll keep the looks and get the strength of the weave underneath.
Still won't solve the problem of the frame design though, but perhaps you can have a stronger one of those made too?
_________________
MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball