you can TRY, and get 90%-99% success
will the surface move? under the fiberglas is there enough support preventing the pan from flexing? Then, will the cement stick when tiled?
On fiberglass, I am aware of very few people doing it, but not what happened afterwards. The big question is how well it holds up.
If I had to, or if i wanted just to try tiling onto fiberglass, I'd tile using only epoxy solids (no portland cement content) grout as the setting bed and as the grout. Epoxy can handle flexing and stresses very well. The project reminds me of someone who has his own little boat that he works on as he choses since it's his and he doesn't have to answer to anyone as to why he did this or that or attempted to stretch the limits of "the envelope" meaning what is or is not done by professionals.
1. The setting bed has to stick; 2. under the surface (the pan) it has to be stiff enough that no tiles crack later from micro movement or from a lack of support. Large and thick high quality porcelain tiles are strongest; they don't require any significant support under each one individually, but then the grout lines take the micro-movement. Where to cut them into the right shapes so they best fit a curved surface is not describable over the internet. The thinner or the cheaper the tile material itself, the more likely a tile will crack. How disappointed will you be if it doesn't hold up over the years?
david
p.s. It's not in the category of recommended surfaces to tile onto.