I had a fully upgraded one for bounty hunting until R* stole it :\
so we aren't allowed to keep one, yet we can still buy its insurance?
if this is true, which i will b testing, then it is like the tow truck, u insure it and the destroy it so that it is stored at MMI and u just call them anytime u want it then destroy it when ur done. works like a charm, same as any other police car u may have managed not to lose.
ok update, this does work and it cannot b stored in the garage but u can destroy it and store in at MMI, call MMI when u want it and destroy it when ur done, only downside is it cant be duped bcus it disappears if u r n the garage with it.