As far as I know the c partition is always the whole disk, regardless of what you have (or don't have) in MBR or GPT. If you want to image the whole disk with dd, you'll reach for rsd0c, always.

But I still don't understand what distinction you're trying to make. Yes, disklabel partitions don't show up in MRB, just as GPT partitions won't. No surprise that the three different partitioning methods aren't exactly aware of each other.