Seen a pic of one with a tow bar fitted so would say yes it could tow a very small trailer ,as to your dog if he/she is happy to sit on your passenger seat then it will fit in a 5,unless its a great dane
If you want to tow a trailer you will need to buy a Jap import. UK cars aren't rated for towing at all so it's illegal to use it for towing anything. You can get away with it with Jap imports though - something to do with the SVA test I think...
Towing is to with the age of the car. I think pre-1997 cars can tow up to 500kg?? After that, you can only tow up to the manufacturers specified towing limit and Mazda didn't specify one for the MX-5 so the towing limit is effectively zero.
