The "rented to others" exclusion has been around forever. It's not surprising to me the crowd at the Allstate office wasn't aware of it, as there is plenty they don't know when it comes to actual policy language.
Florida has a "dangerous instrumentality" doctrine that makes owners of cars, golf carts etc. liable for damages caused by permissive users. You can always pursue the renter for damages you pay as a result of their negligence, however good luck with that.
Some insurance company's sell a policy to cover non owned vehicles but I don't know who they are. Don't think Allstate , SF, etc. do but might find somebody.
Bottom line, it's a mess!!
http://abnormaluse.com/2010/10/judge...dangerous.html