Not OP, but I could see the vision. Right now OpenRouter is developer-paid with no way to pass costs onto the end user. Stripe could help bill and meter at the user level for per-user billing. I would also think more SDKs/easier ways to add AI features (paid for via openrouter) into apps and websites.
OpenRouter already supports this, requests return their exact cost.
Open router supports per-request usage metering, as well as per-key. Building user-level metering from this is trivial, and there are many implementations of this out there. Supporting per-user first-class is trivial considering Open router already has per-request and already supports per-request attribution. How important that is, is a big question, but definitely not something that requires Stripe to build.