The ideal ski for crud and big-mountain skiing won't have alot of sidecut. It will track through rough snow much better than your B5's, and therefore be a good contrast. I have found that every one of the "turny" all-mountain carver skis gets bounced around more in rough snow, no matter the condition. This includes the Volkl AC4, the Nordica Jet Fuel, ect. Even though these skis are 82mm underfoot, they just don't track like the skis with a larger turn radius. I would call them "B5's big brother". Given that you will be skiing this in more of a backside, wide-open setting, I would get away from something that has a similarity to the B5's smaller turn radius.
When I had the opportunity to test many skis this spring in variable, cruddy conditions, the point that, for me, was most obvious was that skis with alot of sidecut, while moderately successful in cruddy conditions, just don't perform like true crud skis. This would include the Volkl AC4 and Nordica Jet Fuel. The sidecut that makes them so thrilling (for an 82mm ski) on the groomed takes away from their crud prowess. To put it simply: the tip gets bounced around much more on these skis than it did on the "crudbuster" skis I tested (the iM82, the iM88, the Outlaw, the 777). The 82mm, all-mountain carver skis are going to behave, more or less, like a wider and slightly less turnier B5, which probably isn't going to offer you the contrast that you desire.
Don't get me wrong: the 82mm all-mountain carvers are great one-quiver skis. But, given the choice of a 2-ski quiver, I would go with a narrower carver (iXRC 1200, Ripstick, Superhape, or B5 in your case) and compliment it with a ski with at least a 17.5m radius and 82mm wasit (88mm if I weighed more). Less radius is better when it comes to a crud ski: the 777 (21m radius) tracked the best of them all, followed by the iM88, Outlaw, and iM82.
I simply haven't been on a better ski than the 777 when it comes to crud: it is more stable and beefier than the iM88, and nothing gets in that ski's way. No deflection, no speed limit. The drawback is that it is barely functional at low speeds or with a less-than-skilled skier onboard, and with such a big turn radius, it won't turn unless you direct it to (which is the whole point).
The iM88 gets away from that slighly by being a bit softer, more forgiving, easier to turn, and easier to ski at slow speed. Unfortunately, there is a slight reduction in top-end stability and smoothness in crud. Still, there is plenty of beef for anyone in that ski. But if taking one run on the 777 and one run on the iM88, you will see that the 777 is beyond the normal boundaries of "smooth and stable" that we normally associate with typical freeskis-it is up there with a race-stock GS in 185. I haven't skied the Stockli offerings enough, but I bet they offer something similiar.
Anyways, to sum it up, I think you will do well to get a contrasting ski to your B5. It is fun to turn, but it is also fun to relax and just blast through the challenging snow!