Hi Dimitri,
Figuring out this BAdI is still on my to do list but as I currently don't have a sandbox and a developer key available, how about an alternate approach:
You can implement BAdI HRBAS00_GET_PROFL and add a check on the SY-TCODE to the standard implementation code. In case the SY-TCODE is 'IW51', 'CATS_DA' or 'CAT3' you can add structural authorizations to all Persons to the users view.
This should effectively give them structural access to all Persons in these tcodes without impacting their authorizations in the other tcodes.
Hope that fits your requirement.
Brent