Unfortunately Ultimate Backup cannot do this.
This is because highways aren't broken down into their own Zone on the map, they are just part of the Zone they are in.
On my map I just have State Police spawn only in LS County and Highway Patrol spawn only in BC.
You SHOULD be able to add an entry to manually spawn whichever car you like, by adding it to the ''SpecialUnits" xml file in the Ultimate Backup folder. Then the custom unit will show up in yellow at the bottom of the menu.
The XML file itself has instructions on how to do this.
I know it's not a perfect solution, especially when you can have it spawn specific units just about every where else (you can have a Stab City specific unit, but not a Highway only unit!). But at least this way you can get what you want, just have to remember to call it separately.