Sure...I've done it within the length of time that was in their normal Park & Fly rates. I just told the front desk what I was doing. Whatever their process is...giving you a card to put in the dashboard, or just logging your license in their system, or...nothing at all (yes I've been places that do nothing)...doing the "park" part first generally isn't a problem at all.

Always places where the car just sits in an open lot. For hotels with secured indoor parking, you might want to call first, but I don't see why they wouldn't do it.

The MCI Airport Marriott occasionally has 14-day Park & Fly rates for under $100/nt. It's a basically a free elite-qualifying night if you need it...and it's actually a fairly decent airport hotel with full buffet for Golds/Plats. The nearby 4-points sometimes has a 7-night Park & Fly for around $60...I've used that one during SPG promo periods without staying in the room.