Actually, the DSN model would work fine for a manned mission to Mars. You're never actually behind the Sun (well it might be possible, but it would be for less than a day). There is an issue where the Sun-Earth-Probe angle drops down to around 3-degrees (so, close to behind the sun), because of radio interference from the sun, but thats about a week long period that you could probably get away with. The biggest cause of comm issues at Mars is Mars itself. Fortunately, all orbiters have an Electra package that allows them to act as relays for each other and for surface assets.
Relay systems are actually more useful in the Earth-moon system at this point. A Lagrange point relay would be important for a far-side base on the moon, or a lander on that side. Earth orbit is where the biggest need for relays is, because the Earth is always in the way for LEO assets. Thats why we have TDRSS.
The biggest issue right now is simply the load on the DSN. Its underfunded and its hard to get enough time on it.