There is not a single crewed vehicle in operation or even planned that supports spacewalks. If it's not next to the ISS, people aren't going to be able to fix it.
At best we could send a robot, which would probably be more easily operated from the ground.
There've been several proposals for an Orion-mated orbital module with an airlock, as well as an ESA one based around their ATV. After all, the eventual goal of Orion is to allow deep-space missions to asteroids etc.
Even coming up with something from scratch is unlikely to cost the $10B we'd lose from a failed JWST.
