Going by my experiences trying to make volumetric texturing in Bryce long ago, these queries arise:-

• A volume would seem to need a complete closed box or bag of mesh polygon faces to define that volume.
• Must that bag or box be all within one group (= one prop, or one jointed part of a character)?
• Must that bag or box be all within one material?
• What happens if that bag or box includes any faces with loose edges?
• If I apply volumetric texturing to a whole character, what happens at the joints? Are the joints treated as split face-edges?