An easy approach can be using SXDA-LSMW, one example here
I know this is for creation, but to update is pretty much the same, you will need a file with all the BP GUIDs (E101CRMXIF_PARTNER_KEYS-PARTNER_GUID) and define a constant on the field E101BUS_EI_STRUC_CENTRAL-CENTRALBLOCK .
Anyway I never tried the MASS, but looks like you can acomplish what you want:
MASS->Object type(BUS1006)->Table name(BUS_MASS_BUT000)->Fields(BUS_MASS_BUT000-XBLCK)
Cheers!
Luis