Assuming equally cared for edges, my personal feeling is that as a group, old pencil skis are a hair better than more modern deeply sidecut skis on the straight line icy traverses that you asked about, but the difference between the two general designs in this particular skiing situation is really quite small.
The deep sidecut of modern skis gives makes them want to carve back up the hill unless you actively modulate edge angle and fore-aft pressure to keep them going in a perfect straight line while on edge. In my experience, these required microadjustments cause the ski to experience microscopic releases in its grip on the surface, resulting in a bit of an impression of "uncertainty" in the traverse, and that you might have to pay a bit more attention if you are on shaped skis.
When shaped skis first came out, had exagerated sidecuts like 115-60-112 and were extremely stiff (eg, red SCX monoblock), there was concern that a phenomena called "bridging" could occur on very steep slopes. Bridging is what would happen if the tip and tail of such a ski dug into the surface, but the ski was so stiff, and the slope so steep that the waist would not deflect downwards enough to contact the surface. This was essentially a theoretical concern and made moot by in the next generation as the skis were made softer.
On the other hand, there is a major benefit to modern skis in the situation we are discussing (ie, an icy traverse). The shorter lengths and generally softer flexes of modern skis tend to put more pressure on the surface directly underfoot, and this helps you dig into hard surfaces a bit more, and hence hold the traverse better. OTOH, if the surface is a bit uneven (rutty, chunky, etc.), then the longer ski tends to smooth out these irregularities a bit better than a shorter ski.
So, there are a bunch of positive and negative factors going on simultaneously, but IMHO, they almost completely cancel out in this particular situation, perhaps leaving a slight edge (pun intended) in favor of pencil skis (again, in just this one case). However, the slight advantage here does not come even close to making up for the overall benefits of shaped skis in almost all other skiing situations.
While the above discussion dealt with these two very broad classes of skis (ie, old vs new), OTOH, I think you will find much larger and quite noticeable differences within the highly differentiated world of shaped skis in the situation you asked about. The most extreme (and obvious) example would be to compare a modern wide powder ski with a modern narrow GS ski on the proposed icy traverse.
Just my $0.02,
Tom / PM
[ October 03, 2002, 08:51 AM: Message edited by: PhysicsMan ]