This actually shouldn't be a rule by the server. Since this is a Roleplay server, LCPD should be the one who makes that rule and should be able to punish for it the freecops who refuses to follow it. LCPD have already got that rule only inside the department, this rule does not apply to the freecops in the server because of some unknown reasons, from my side.
If we want to make this happen in the Server, LCPD should have the permission from the Server leaders to control the freecops, because that's actually simple.
As Mane said, making a server rule for that wouldn't really fix it, this is a roleplay server and players should have the freedom in doing whatever they want (unless they break server rules), but since this is a work/job in game and have a official group which should have the power over the freecops which is LCPD, then the LCPD itself should fix this or keep it as it is.
In my opinion, LCPD should be the only one able to punish the freecops in the roleplay side (Admins should be able to punish players only if they break the server rules), by roleplay side I mean the usage of the non-marked vehicles for chases, reckless driving and breaking the law while on duty...etc.
I don't support making a rule, but there is another way to fix this, by roleplay.