Give hulls for which custom paint jobs are available a new 'paintjob' rigslot. Paintjobs are new items that act like rigs and can only be applied to the relevant hull and into its paintjob rigslot. Removal results in destruction.
BPCs for these items is a different matter, but they could be on market, NeX or from LP stores. They could require Aurum in the price, or during construction.