Still a long shot, but it would improve your odds if you gave some idea what this volunteer laborer could expect to see and experience during the trip. "Come share A, B, and C with me at my expense. All I ask is that you handle the language stuff for us" is much more appealing than, "I'll give you a chance to work for nothing so you can have the practice". We're all surrounded by endless opportunities to use Japanese all the time without getting paid for it anyway; presenting it as a busman's holiday won't get you anywhere.