Turtle Toad,
Just noticed that you asked specifically about CARS - not RVs.
Yes, I checked out National Interstate before, and they do offer the coverage for 'Companion Autos' if you ALSO insure your RV with National Interstate. The only problem was that the National Interstate policy for the RV was very expensive, so once I crunched the numbers, I found it more cost effective to insure my rig with Progressive, then just buy the Mexican 'full coverage' insurance through the Mexican insurance company.
Adventure Mexican Insurance gave me the full scenario so I was able to crunch the numbers (they could give me a Progressive and National Interstate quote) Also, National Interstate does not seem to have as large a presence as Progressive (claims infrastructure) so I felt more comfortable with the Progressive policy.
Unless your car is worth over $40K, the full coverage Mexican insurance (Mexican carrier) has decent coverage, reasonable deductibles, and decent rates. It's when you have an RV worth more than $50K that the Mexican insurance carrier programs start getting really expensive and the deductibles get ridiculously high.
So our experiance was - get Progressive US RV insurance, and get full coverage Mexican insurance through the Mexican carrier.
Call the guys at Adventure so they can run the numbers for your scenario.