There are actually quite a few issues with texture management I have to admit, so you are correct to question this.
While materials should already be disposed of properly in most circumstances, I was ignorant so far to the texture situation itself.
Improvements are planned for an upcoming update where materials automatically get rid of their textures. But this will unfortunately not address "Set mesh texture".
Proper texture management in the future (before 1.0) will have the aim to dispose of any texture in memory that is not currently used by any object and also probably provide shared textures for non-instanced objects.
Speaking more generally I don't know if it's the best approach to load/dispose the texture every time for a supposed LOD change. What kind of objects/behaviors are you trying to apply a level of detail system to like this? And are you using large textures?