First of all I have never used laminate hardboard..or know what it is for that matter but do you think it is strong enough to hold the shelves of melamine(you already commented on how heavy melamine can be so I wont repeat it..oops I just did..)? Thats definitely something to take into account. Second, most building materials are capable of handling pretty high temperatures..otherwise there would be a LOT more fires than there already are...I've seen some back heat using peg board which worked fine.. Third, you're using BOTH belly and back heat?? That seems like overkill to me, as long as your room temperatures are not in the low 60s you should be able to get by with just belly heat(I use 4" flexwatt on my racks and keep it at around 98, which gives pretty good temps in the tubs 90* hot spot, 88* warm side 79*cool side) and I dont even have a back to my rack.
And PS. an oil filled radiator to keep the room in the mid 70s is really not that expensive, I just picked up 2 a few days ago(cause I live in a craphole apartment that wont heat up worth a darn..) for $80..