Ok, I ended up debugging the whole odious code related to address subscreen field modification on both sides (BUPA and CAM) to understand just how the expletive thing is supposed to work and solved this nonsense as follows:
1) defined in TA BUS2 two Customer fieldgroups, one for each set of the worthless fields:
798 "Adresse: EXTENSION1" containing:
ADDR1_DATA, EXTENSION1, X
ADDR1_KEYW, EXTENSION1, space
799 "Adresse: EXTENSION2" containing:
ADDR1_DATA, EXTENSION2, X
ADDR1_KEYW, EXTENSION2, space
2) assigned both fieldgroups to view BUA110 "Adresse: Daten (Organisation und Person)" in TA BUS3; thereafter the groups become available for configuration in SPRO, BUSO and BUSJ;
3) in order to pass the configuration to CAM one needs to:
- go to the expletive screen and find out what little odious Group4 has been assigned to the worthless fields:
- go and do implicit enhancement at the end of SAPLBUA0 FLDGR_CONVERT1, mapping from the little odious Group4 to BDT fieldgroup:
if e_rc = gc_x .
case ie_fldgr .
when '046' . "Dynpro Group4 of Address: EXTENSION1
ie_fldgr = '798' ."to BDT Fieldgroup
clear e_rc .
when '047' . "Dynpro Group4 of Address: EXTENSION2
ie_fldgr = '799' ."to BDT Fieldgroup
clear e_rc .
endcase .
endif .
- and another implicit enhancement at the end of SAPLBUA0 FLDGR_CONVERT2, mapping from BDT fieldgroup to the little odious Group4...
if e_rc = gc_x .
case ie_fldgr .
when '798' . "BDT Fieldgroup for Address: EXTENSION1
ie_fldgr = '046' ."to Dynpro Group4 of Address: EXTENSION1
clear e_rc .
when '799' . "BDT Fieldgroup for Address: EXTENSION1
ie_fldgr = '047' ."to Dynpro Group4 of Address: EXTENSION1
clear e_rc .
endcase .
endif .
And that's finally the whole expletive deal then, hopefully... The enhancements could maybe have been replaced by BDT fieldgroup specific field modification FMs assigned when defining them in BUS2, but I wasn't about to take any expletive chances with something so advanced and refined...
I'll report this to OSS when I've calmed down next week 
cheers
Janis