I'm building a DE army right now, so I'll have to give this a try. I'm thinking of a modified version from WMA: based facing the short edge, no support, 0 attacks when fighting on the flank or rear, or when fighting in terrain (to a 1 attack per unit minimum). They can move through terrain, but are basically helpless in it.
I was thinking at first of trying this out as an elite unit (0/1 per 1000 or something). But now I'm wondering if the more cautious upgrade might be to change the entire DE Spearmen unit.
Hear me out.
The advantage of a phalanx is that it's very powerful on attack. The disadvantages are: the narrow frontage makes you likely to be out-flanked, a situation where phalanxes are almost defenseless. They're also useless in heavy terrain (in fact, banned from it in WMA).
If the phalanx is an elite version of a conventional troop type, then most of those downsides don't come into play, or can be easily mitigated. Put your phalanxes in the middle and your spearmen on the flanks. With only a few phalanx units, your battle line is still pretty broad. Keep them in open terrain; again, when it's required, just use conventional infantry.
Now let's look at what should happen if you have only phalanxes and no conventional spearmen. First, your battle line is necessarily narrow. And what do we put on the flanks, or send into terrain? Crossbowmen or witch elves, I'm assuming. With their lower armor ratings, neither is well-suited to the purpose. So what do you get for all this? Another attack unit. Dark Elves already have LOTS of attack units: Cold One Knights, Dark Riders, Witch Elves. It's nice and very dark elf-y to be able to rush your whole army forward, but I'm not sure it actually strengthens their hand any, especially given the limited number of leaders.
Again, this is all theoretical. I haven't actually played a game with Dark Elves in either mode.
Basing: the only dark elf infantry figures are corsairs and spearmen, both of whom are easy to base in either direction. How to mount them? At a guess, you could use litko 20mm square bases with 3mm "flight base" holes, and conventional bases made of "flex-steel". Put magnets into the holes and then you can stick or unstick them for "official" and "fan-created" units.