I would strongly advise against traveling with and depending on a card so close to its limit. Not only won't Hertz extend without taking an additional authorization, it may well hold a good deal more than your anticipated rental in order to account for damage.

The same is true for many hotels, which will place a hold for the amount of the room and a set amount for incidentals. If you have this happen with Hertz and then a couple of hotels for short stays, it is easy to run over your credit limit and wind up stuck with no access to credit.

Better to work with the card issuer (bank) to inrease your credit limit, even temporarily, for purposes of accommodating this.