Why not have an GUC+ (insted to choice 4 x GUC, you can choice 2 x GUC+).
The GUC+ give you access to upgrade at ticketing to J (no class restriction).

Or to use 2 x GUC to avoid the class restriction.
This will also lower the "usable" # of GUC (and leave more GUC seat for "normal" GUC PAX).