like i said, i'm 100% certain that it shows up in your download list once you redeem the code. i know for sure because i did this for my rb gameshare group. i went out and purchased the game, signed into the gameshare group and redeemed the code. then it was available for everyone to download and, subsequently, play in rb. i tested this and played the ac/dc songs in RB2 with a friend in my gameshare group.
you never even need to use the disc to do this, it's that simple. of course, you won't get whatever joy might be found on the disc, but who cares? having the tracks in your RB library is much more important, imo. i still haven't played the songs outside of RB2.