Stuck inside MPCV for 20 days? Ow!
MPCV is a capsule, and like all capsules, it's designed to be a safe place for the Astronauts to sit during the exciting bits and to re-enter. For anything else it, like Dragon/CST-100 etc, would be a real drag to be stuck in for days on end and probably messy too.
All the (admittedly incoherent) ramblings on the Interwebs that mention MPCV going to deep space involve a habitat module for anything beyond Earth orbit. The one exception I've found seems to be the lunar orbit plans for SLS-1 (unmanned) & SLS-2 (manned) where it'll be 3ish days to the moon, 3 or 4 days in lunar orbit (Doing what? No-one's mentioned any kind of instrument package or the like) and then another 3ish days back to Earth.
I'd like to note that, while SLS, expensive as it will be, is essentially all-new, it won't, itself, be going beyond LEO. It will give enough push to get the MPCV and its service module/upper stage on the way to the moon. It's like the Falcon Heavy in that respect.
I'd also like to note that, despite the very large amounts of money being thrown around, there's no talk of an all-new upper stage to take people BEO. It's just the old Delta IV upper stage design (possibly the design/wiring/electrical systems will be rejigged).
If SpaceX does ever send a manned mission to Mars (one of Elon Musk's fantasies but hey, so was SpaceX itself) then they'll have a Dragon capsule or two but the crew will have, I'm sure, a dedicated habitat module.
I'm sorry to say that while NASA doesn't build anything itself, its funding has been hijacked by the Senators/Congresscritters to keep people employed even though they'll be working on something so expensive that, after Falcon Heavy flies 2 or 3 times (yes, I'm making an assumption that everything will go ok but that's not unreasonable, I think) the Senators/Congresscritters NOT on the Spaceflight committee (ie those who don't have people employed making/designing SLS) will object to the enormous cost disparity between the two rockets and vote down continued funding for SLS in 2014/2015.
Making deepspace commercially viable is still about 10-20 years away, IMHO, because of 4 things - initial cost (still too high), reliability (no-one's done deepspace commerical activities so this is going to be a low number), capability (we still can't put much into LEO, let alone deep space) and time (asteroid mining, for example, is a case of sending off a mission and getting the asteroid (or parts of it) back in 20 years or so).
Still, it'd be nice to spend a billion dollars on a mission to send a mining mission to (6178) 1986DA, a platinum-group metal-rich asteroid, break off a few hundred tonnes and send that bit back to Earth. At a current cost of 1,400 dollars per ounce, even a hundred tonnes of platinum would bring in 4.9 billion dollars.
Worth waiting 20 years for...