I think FRP would be fine, it's those flat plastic panels used in commercial utility rooms, bathrooms, and kitchen walls, even showers. However, it's not originally intended as flooring, so just make sure your floor is nice and solid (any bounce may make it crack).
My concern is how you would seal the seams in the middle of the room - unless one sheet will cover your whole floor, in which case you only have to seal where it butts to the walls.
Since you're wanting to be organic, I'd recommend finding out what type of plastic is used in the Glasbord FRP. The F is fiberglass, the P is some sort of polymer/plastic. Make sure it's not #3 (PVC) or #7 (Miscellaneous) as they have those added softeners like phthalates and bisphenols (BPA) which can leach out in heat. Usually not a big deal but since you're outdoors, and wanting to be organic, figured you'd be interested. #2 (HDPE), #4 (LDPE), and #5 (Polypropylene) are 100% safe, and #9 (ABS) is probably safe but not a lot of tests on it (it's what Legos are made of).
My concern is how you would seal the seams in the middle of the room - unless one sheet will cover your whole floor, in which case you only have to seal where it butts to the walls.
Since you're wanting to be organic, I'd recommend finding out what type of plastic is used in the Glasbord FRP. The F is fiberglass, the P is some sort of polymer/plastic. Make sure it's not #3 (PVC) or #7 (Miscellaneous) as they have those added softeners like phthalates and bisphenols (BPA) which can leach out in heat. Usually not a big deal but since you're outdoors, and wanting to be organic, figured you'd be interested. #2 (HDPE), #4 (LDPE), and #5 (Polypropylene) are 100% safe, and #9 (ABS) is probably safe but not a lot of tests on it (it's what Legos are made of).