Interesting; that's not common to find, IME--very few non-owner's policies include property damage coverage.

I'd be slightly leery of relying solely on a credit card for collision coverage. Call me overly cautious, but I've personally seen credit card CDW claim denial letters because of minor technicalities (renter didn't report the damage to their card company within 48 hours, etc.). Credit card CDW is not (AFAIK) a regulated product and is subject to the whim and goodwill of the carrier, and they're of course incentivized to pay out as little as possible, whereas there are laws and/or case law in many states obligating traditional auto insurance providers to cover rental cars. If you can't obtain an auto insurance policy that covers property damage, then I'd feel a bit better about having two sources of coverage--your credit card plus a paid CDW product like one from TravelGuard, etc. You just don't want to be out $30,000 when your Taurus hits some black ice and spins into a concrete wall and your credit card company finds some technicality to get out of paying.

Back to the OP's original question--

You might have some luck if you talk with the branch manager at an Enterprise branch (on- or off-airport). They're given a lot of authority to do pretty much anything they want with rates, and they're highly incentivized on fleet utilization. So if you can guarantee them a car off their lot for lots of rentals, they might be happy to come down on the rate.

You can also talk to managers of other companies, but depending on whether they're corporate or franchised and the corporation's policies, they may not be able to do much for you (if rates are set centrally, for example). You'd likely have better luck with managers/owners of franchised companies, since your rental rate goes directly to the business's bottom line and the owner's pocket and they're more likely to negotiate. I didn't look, but it's likely that at least some of the agencies at FSD are franchised, since it's a smaller location.

OTOH, sometimes the local manager's idea of a "good rate" is a "nice, fair" $30/day when retail rates are $40...and (especially with coupons and discounts) you might be able to find a rate online of $20/day. So talking to someone even if they have the best intentions won't always get you the best deal. It is worth talking and seeing what the options are, but don't forget to search on your own, too.

For example, I just did a search for 3/1-3/10, and the best regular rate in the market now is $280/week, total of $444.75 for 10 days (with Enterprise). The Enterprise branch manager might think a rate of $250/week, total of ~$410, is a good discount for you, but I was able to find a discounted rate of $238/week with Hertz plus an extra coupon for a total of $338.81--more than a hundred bucks off the cheapest retail rate.

It could be difficult to get that much off your rental, even from a branch manager. My own personal experience says that rental industry managers often tolerate low rates from online sources as a necessary cost of doing and attracting business, but they don't actively encourage low rates (disregarding that $35/day is not all that low, of course), so getting a low rate out of one of them in person is a little harder. Perhaps not impossible, but it depends on the person you're working with.

Also, if you go with Enterprise, be sure to get something in writing from the branch manager. They tend to rotate branch managers between locations every six months or so, and you wouldn't want your deal not honored because a new guy came in to run the place. Management at other companies is typically more stable.