There is code (even in stock renegade) that allows the Commands->Set_Is_Rendered script command to work for vehicles. Scripts 4.2 expands the command to work for other things but it works for vehicles even in stock.
I have no idea whether an object hidden by Set_Is_Rendered will be targetable or not (or what other side effects using Set_Is_Rendered will have) but its there and usable.
Jonathan Wilson aka Jonwil
Creator and Lead Coder of the Custom scripts.dll
Renegade Engine Guru
Creator and Lead Coder of TT.DLL
Official member of Tiberian Technologies