Y'know - even if it is a interlined ticket, you have to clear customs in BOM. So you have to get your bags off the belt and check them in anyway again - not sure if you can just hand them off to the 9W reps at the international terminal.

If I transit through BOM I try not to check anything at all .. one rollaboard and one laptop bag. So ..

And anyway both BA and 9W have international flights to MAA (9W has BRU-MAA now which you can fly instead of BRU-BOM).