The recipe is something like this:
- Remove the line in C4Def::CompileFunc in C4Def.cpp that connects the Defcore value to the internal engine variable
- Remove the internal engine variable from C4Def.h and replace all users with GetPropertyInt(P_Foo). For non-integer values it gets a bit more complicated. See C4PropList.h for the APIs to use.
- Add one line each to C4StringTable.h and C4StringTable.cpp for P_Foo, mirroring the other P_* values
- Add the property in question to tools/convert_cr_to_oc.py, mirroring Collectible or Grab/Touchable and run the script in planet/.

Another problem is that a lot of engine functions pass around C4Def* pointers, which makes deriving definitions somewhat useless. I don't have a better idea than changing those to be C4PropList* and writing a custom check that the proplist is a definition (pPropList && pPropList->GetDef()) instead of making the binding templates do that.