Hi Boris,
if I remember right, you're using the customer like respectivly the vendor like UIs? In that case you could change the defaulting of the BP grouping with some lines of ABAP. Take a look at method in "DERIVE_BP_GROUPING" class "CL_MDG_BS_BP_HANDLER". This one defaults the BP Grouping according to the given BP ID.
Or in other words:
The method determines the current BP ID and BP grouping (lines 24 to 48). If the current grouping is initial (that's of course always the case during creation) it uses the current BP ID (that is during creation usually an internal ID) to determine the related default BP grouping for internal numbering (lines 50 to 59) . The determined grouping is then assigned to the BP grouping (lines 63 to 72).
Now, if you'd like to change this default behaviour according to your requirement, you could consider implementing a custom enhancement of the method, e.g. using the implicit enhancement option at the method's end. Based upon the object's type (customer or vendor, you might find lines 21 and 22 helpful for the differentiation), you could define your own default grouping and overwrite the value determined by the SAP default code (e.g. modify lt_write and call io_write_data->write_data one more time).
Best regards
Michael