I'll assume you're desiring a reasonable WB to have a good starting place for artistic interpretation.
Here's a couple ideas:
1) Use a "Delta Gray card" and cut it in half, put it in a Ziplock, throw in your bag. Almost any camera shop, even independent ones in small towns have them and they're like $5-$6.
Place it in the scene. Done.
2) Most camera bags have a gray interior, some have a color cast, test your bag, rain cover, etc. If its "accurate" enough, then you're there. Just take a picture with your bag in it in the same light.
3) Use a gray card, white sheet of paper and do a custom white balance prior to shooting. Some cameras become warmer/cooler based on whether you use gray or white.
If you notice, I'm not focusing on it being "perfect". In landscape, the tone, colors, balance, etc. are all part of the artistic interpretation.
If you were doing product photography, I would recommend getting a Gretag Macbeth ColorChecker & EyeOne, calibrating your camera and using it in the scene. Along with Sinar Braun or Profoto digitally calibrated strobes!
Enjoy the outdoors!!