I was awarded a BCD for being UA from the Marine Corps. With an RE-04 reenlistment code, can this waived by the National Guard? Any advice would be appreciated.

Can it be, I am sure anything can be waived at some point, but I doubt it will ever be.

I don't know but it is probably worth a try, especially if it has been a while since the BCD. There have been a lot of changes recently in minimum standards.

I was awarded a BCD for being UA from the Marine Corps. With an RE-04 reenlistment code, can this waived by the National Guard? Any advice would be appreciated.
You would need to look into a "Discharge Review Board".

The Dept. of Defense has uniform policies, pro-cedures, and standards for the review of discharges or dismissals under 10 U.S.C. 1553... each branch of the Armed Forces has a review board, but I'm not sure of the exact procedure to appeal your discharge.

Contact the JAG office closest to you and ask them... or simply ask your NG recruiter to do the research for you... I'm certain he'd want to help.