One problem I see in the near term is that NASA plans to use Dragons as lifeboats, so I’m not sure when one would become available on orbit.
[Update a while later]
Actually, I think that a cargo Dragon meets the requirements for this much better than a crew Dragon. It’s an on-orbit mission only, so there’s no need for couches, which just take up room. It can’t be used for a lifeboat, because it has no docking adaptor (at least currently), so NASA wouldn’t miss it. Even a Dragon V2 would need an ECLSS upgrade, so might as well just put it in the cargo version. It would have a lot less value to NASA than a V2, so it would be easier to get it from them. All they’d be giving up is the cargo return (which they could even get when the mission was over, months later, if they wanted).