Rayman,
I addressed this same question back in August (
Plastic Bed Wood )
I did some research on this idea a few years back. Contacted all of the manufacturers and got data sheets, etc. The only problem with using plastic lumber is that it tends to sag if the space between supports is too large. Using 3/4" lumber (which would match the thickness of the original bedwood) you need supports on a maximum of 16" O.C. (12" O.C. recommended by some manufacturers). The space between the bed sill supports is much greater than this. If you don't have adequate supports, your bed boards will sag between the supports. If you could work out some system of additional support on the underside, I think plastic lumber would make a dandy bed.
