I'm gonna say, probably not. Modern hard plastics, aluminum, and steel are basically non-reactive; leather and wood aren't. Leather is tanned...which comes from tannins...mildly acidic substances used to convert the proteins in hides. Some are left over. Wood has its natural oils. And both would probably outgas, especially under the influence of heat, as from a CPU.
Update: I looked at the wood case posted as a DIY. That guy isolated everything by not building ONTO the wood; he's got a metal tray for the mobo. And the tray helps with stiffness, which would be another concern.