Yes it can cause discoloration of the wood over time! You can but a piece of tile to put under the UTH or whatever idea you might come up with! 
Fire hazard, whats that mean?? Lol As far as a fire hazard, i dont think there is a reptile product such as a UTH or heat lamp that does not pose some sort of fire hazard!! The UTH going at full throttle i think only gets to around 120F, I dont know what the flash point on wood or treated wood is but its probablly a lil higher then any heat the UTH is going to produce. That is how we have all ours set up and so far no problems! Hopefully you will hear back from some more people on how they have there set ups done!
Good luck
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Mike & Wendy 
2.0.0 Siberian husky, Jackrussel/schitzu
0.0.1 Ball Python
1.0.1 Leopard Gecko
1.0.0 Beardie
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