Hi,
the BAdi mentioned by Martin should be triggered. I already used it for the BP POWL and it worked.
You can also just create your own feeder class (use the standard feeder class for business partner as superclass --> Feeder is SAPSRM_FEEDER_BUPA_SUP ). Then redefine the methods you need. If you just want to restrict the selection results, this might be too much of a change... but it certainly helps performance wise cause in the BAdi you often do you have additional selects on the database since sometimes not all required information is available.
Markus