For anything electric, UL's tests include some sort of overload/short circuit test, during which the enclosure is monitored for temperature. The temperature, in genreal, is not allowed to exceed a value that they associate with being likely to ignite easily-ignitable materials, such as wood or paper.
There are exceptions; for example, we have all seen fixtures marked as not being suitable for use inside dwellings. This marking is there because, in the absence of free air flow, these fixtures do get too hot for fire safety.
As for the insulation requirements, the UL tests also include some sort of environmental testing, which may include running at a higher than normal temperature for an extended period of time. The higher-rated insulation is required to protect against premature breakdown of the insulation over time. This has nothing to do with failure effects; quite often, a failure (such as a bad ballast) will get hot enough to damage even specified wiring, and some length will have to be replaced.
Unless the fixture's directions, or marking, prohibit directly mounting the fixture against the ceiling, there should be no problem mounting them to a wood shelf.
Even at 90 degrees (centigrade), wood is in no danger of being burned, charred, or ignited.