Just let them check your bags from BRU to IAD because you have to recheck it anyways and when u hit IAD the first time just go to a counter to let them tag it for your IAD - DXB flight so it will not go with you to BOS ... not really sure if it works but it worked for me when I did FRA - ORD - CAK - ORD - ATL, I had two seperate tickets one from FRA to CAK and one from CAK to ATL arrived in ORD went thru customs and told the recheck agent to tag it just to ATL then ORD - CAK - ORD - ATL .... since the bag was all ready labeld to CAK in FRA... they took the old tag off and put ORD - ATL on it....hope it will work for you