I recently repaired a friend's plastic fairing on a Fazer 600. I drilled a small,hole at the end of the crack, to stop it spreading. Then I cut some light metal mesh, into a strip the length of the crack in his fairing. About 1.5" wide. Then pressed it into the plastic, along the crack, on the inside of the fairing, with a flat head bolt tightened onto the tip of a hot soldering iron. The mesh stops it flexing and re-cracking. Just filled and repainted the outside. Seems to be holding up.
Don't know if it will stand up to the rigours of a seat pan, but maybe worth a try, with first a layer of glass fibre, then the mesh, then another couple of mat layers over the top?