Powdercoating is not going to work, as the temperatures necessary for melting the powder are certain to exceed the temperatures for the model. The process works by placing a charge on the blown powder and the opposite charge on the item being coated. If the item has tight clearances between sections, the charged powder will not enter those areas and can be repelled. Your description about not being able to reach it with an airbrush would lead one to believe you have similar complication. This means that using an electrostatic liquid paint system would also be problematic. If you can add a photograph of the difficult areas, it might be helpful.