I can build a QtWebkit based browser, fulfilling your requirements from the technical viewpoint.
Also included, I could provide a site (with top level domain), where to host downloads.
The browser will be portable on all major desktop platforms, but it will require custom development for Android and iOS. I think I can insulate the applicative requirements from the actual Web access/rendering code, then it should be doable.
Another point, I'm not sure about the user model to make Yahoo BOSS API available. From what I can see on their site, it's a subscription based service. Then either we (you?) will provide the subscription fee on behalf of users, or an additional service to automatically register users - and billing them - will be required.
Waiting to ear from you...
Best regards, Ing. Carlo Capelli