the following thread contains some ideas for adding a sternum strap to a G6. i find them very workable.
G6 sternum strap
also, i've added a "poor-man's" pad pocket to the G6. using a NightLight Toros pad (or Z-rest pad), the pad can be very easily held in place by some 1/8" bungee threaded through either 4 or 6 of the webbing loops and crossed in a simple, obvious pattern. the friction of the pad and the fact that it's "sandwiched" b/t the pack and your back, when wearing it, results in the pad "staying put". it simply doesn't slide down. i use very little tension on the bungee. it doesn't pull very hard on the webbing loops.
storing the pad this way frees up volume in the G6. it also carries very comfortable. yes, it does move the pack a bit further away from the back, but no further than with a G5 or Mariposa, plus the pack (and its contents) are generally lighter, so it's less of an issue with the G6. no, it doesn't function as a frame - since there is no waist belt there is no way to transfer load from the shoulders to the hips anyways.
wish my G6 had a mesh front ocket; G5 does, and i miss it. i know. mesh weighs more, but, IMHO, it's worth it.