You could potentially reformat your data to get around this. With CBC/HB, you could code the entire interaction by hand. For analysis purposes, I might be tempted to add a new attribute that indicated the SKU grouping, then create an alternative-specific attribute for each set of SKU's. Then the software could estimate the interaction between the subset of SKU's and the promotional attribute associated with that set since each promo attribute would be fully interacted with the SKU's that pertained to it. You'd need to estimate the SKU Grouping utilities to tie the whole thing together, though, so this might complicate analysis a bit.