Dynamically set PRPS-USRxx fields to be optional or mandatory

Hello

We use some user fields in WBS elements. However, depending on field content we have in PRPS-POSKI, PRPS-PLAKZ, PRPS-BELKZ and PRPS-FAKKZ we would like to dynamically set some specific fields to be mandatory or optional. A solution based on the level of the WBS element in the hierarchy alone is not sufficient for the requirement we have.

In PBO of Program / Dynpro SAPLCJWB / 1460 there is no useful user exit, BAdI or Enhancement Point. The only option would have been to make use of enhancement spot ES_SAPLCJWB_PS, enhancement point  EHP603_FCJWBO00_MODIF_PBO_01 but this is marked with "Can only be impl. SAP internal". :-(

Any ideas out there allowing to solve our requirement without applying a real modification?

Thanks to all,

Renaud