From an old report,
wood heated to around 100°C looses its humidity but doesn't go through any transformation.
at 150°C it looses gaz (carbon dioxyde and others)
between 230° and 270° it transforms into red coal that can burn spontaneously if hot air is blown on it.
At 300° it turns into black coal that needs heat to burn.
So it looks like you're ok if the temperatures stays below 100°C
It says too that the continuuous action of a small heat source can lead to the formation of this red coal, but not if it's intermittent or the wood is heat treated, no temperature is cited.
Here's the report (that's french, sorry)http://e-collection.library.ethz.ch/ese ... 060-02.pdf