You're right that Platinum doesn't get you any automatic upgrade on the Convertible class. That said, as a Platinum you'd at least be able to ask to see what they could do for you. I guess it would depend on a) what they had available and b) how friendly of an agent (or manager) was helping you. Very much a YMMV situation.

I do want to note though that I just took a look at Enterprise.com for OGG and it looks like they only have a basic convertible available. They may have some nicer stuff on-hand, but the only convertible that seems to be reservable is a Ford Mustang or similar (which basically means it could be anything).

Lastly, if you want to PM us your email we can see what we can do with Enterprise if you still prefer them. We might be able to get the rate down a bit.