I like Redfern's idea too, to a degree. One major point it has going for it is that it allows the barrier to exist in Star Trek mythology without its absence conflicting with real world observation. In-universe, the explanation would be that we can't see it, because we aren't FTL capable.
The problem that it runs into is that the Valiant had impulse power only, and the barrier had a real effect on her. I'm not a subscriber to the fanwank that impulse is some form of FTL, crude or otherwise. I can (I suppose) tolerate a warp-capable ship using a warp field to reduce its inertial mass so that rockets can accelerate the ship faster than otherwise. Considering impulse itself to be FTL is a bridge too far for me, though.
In my personal continuity, the Valiant was swept to the edge of the galaxy by the "magnetic space storm", which is an FTL effect in the Star Trek universe, possibly related to the barrier itself, and she wouldn't have been able to get out that far ever, otherwise (with a live crew). In fact, that right there is another alternative: perhaps the barrier flares up in some sort of cyclic pattern related to the magnetic space storms, and that's why we "don't have any evidence of it" at the moment.