I do not seemuch problem with using fiberglass on the roof, however, I don't think it will add much support. For there to be added support, you need to add "structure". A line of longhair fiberglass will do little to add structure, even if you make the new "freeform rib" somewhat substantial.

To compound this, I would want to think twice about "bonding" fiberglass to the roof, as the roof is subject to huge temperature changes, and therefore lots of expansion and contraction, all at a rate different from that of the fiberglass. I think that it would be difficult to make a good bond considering this expansin differential. If a good bond were to be made, it might cause the roof to actually bend and warp.

An idea to make a cross rib might involve making one out of 1/4" plywood cut to fit the conture of your roof, built with some "structure", fiberglassing over the wood with "real" fiberglass, and bolting the unit into place without bonding the unit to the roof.

Other than that you could try fabricating a "fitting" support beam out of light steel, this would probably be your best albeit most dificult solution.