or add candi sugar and call it a belgian imperial stout
Belgian stouts are more about the yeast and not the sugar. Do we call Imperial IPAs that have a sugar addition Belgain IPAs? BTW, I think Belgian stouts are bad tasting combo. The roasted malts and yeast phenols clash, IMHO. I have not impressed with any of the commercial examples I have had.
I think D2 would be a better choice than molasses or brown sugar, IMHO. There are really no flavor contributions from brown sugar.
This is why I always check my initial boil gravity with refractometer so that I can add DME or dilute with H20 depending on my gravity after 5 minutes of boil so that I hit my OG prior to yeast pitch. A little tip I learned from Mike "Tasty" McDole on the BN.