Could a spacecraft use organic materials, like wood or bone or shell or other tissues, as an interface to the vacuum and radiation of space? Would wood outgas and crack up in space? I wonder if life on Earth has evolved materials, without human technology such as metallurgy, that could work for spaceflight. Something which might have evolved somewhere out there.
Yes, and it has been used in the past (See: NASA - balsawood). It was not a significant portion of the spacecraft but it has occurred. You can, in theory, use any material to build a space vehicle, however, mechanical constraints will arise. A mechanical designer would have to evaluate the cost-benefit of utilizing wood and determine the risks of using a non-traditional material.
Space is a brutal environment with extreme temperatures, significant radiation exposure, and vacuum environment. All of these are factors analyzed in FEA simulations by engineers prior to making such a design decision. Since you mentioned 'spacecraft' I'm assuming you're referring to some kind of orbiter, satellite etc. Aside from the space environments, there are significant stress loads (i.e. vibrations, g-forces) that are experienced by the vehicle during launch. Lastly, there may be cases where manufacturing wooden parts prove to be more difficult than more widely used metals such as aluminum.
Like you mentioned, outgassing can be in issue in porous materials because a vacuum environment can cause the expansion of gas pockets. This can lead to material failure where the structural integrity of entire spacecraft is destroyed. As far as evolved materials are concerned, I'd find it difficult for it to naturally occur because the space environment is not immediately present during the evolution process of plants and mammals due to our atmosphere and gravitational pull.
Wood could not be used alone. You need a very good glue for making plywood with crossed fibers and for the assembling of several parts. Wood and glue would outgas any remaining water and would get brittle. A protection against the very intensive UV radiation as well as against micro meteorites would be necessary.
But every kind of wood, glue and varnish used should be space rated. The whole production process from drying of wood to cutting, preheating, pressing, resting should be controlled. Even the humidity of air during production has its influence.
The wooden parts should be tested just like other parts, a vibration test, a sun simulation in a vacuum chamber. You could not rely on experiences made with metal parts. If a different material is used, all tests should be repeated.
Many decades ago, military aircraft were built from wood, but nowadays metals and composite materials are used.