I have found that many hotels, usually with larger gyms, offer memberships or day passes.
I've noticed these too, but generally like you said at bigger hotels where the gym has its own branding and operations. I'm more talking about smaller airport-ish hotels where the gym is generally unattended and has nothing more than a door requiring a keycard. I'm thinking more of an informal arrangement with the property itself... anyone have any experience? Or should I just go down and ask?