Tough to say. I know that Recurly has gone the approach of supporting any number of gateways (http://recurly.com/gateways/). Is there a technical limitation of Stripe that doesn't allow for this?
I was under the impression PayPal does allow charges with their Direct Payment API (i.e. Payments Pro). Are you specifically referring to the fact you have to be redirected to their site for login to do so?