Think the biggest issue would be base models of the avatars. Almost everything we've had revolves around modelling onto those pre-sets. There's very few actual variations involved.
The ones that have stood out utilised transparencies to "cut out" portions of stuff to have a different look (the "scarfs" made from base cloaks and a cut out from a transparency), or with animations as well (the Vecna stlye of floating bandages). But they all still needed the base part there to work with.
So the dress may not be viable with what we have right now. Not only that, but we'd also have the gradual change by SSG over the years where they have slowly added more modesty adjustments, thereby preventing certain looks from being possible. That Silver Flame Blonde NPC crouching in the Orchard was given a shield to prevent being upskirted, even though her "skirt" is part of her armour and she's fully armoured there too (so nothing that should be avoided).
So dresses with such high cuts along it, may not be allowed or requires further adjustments because of previous work performed to sanitize textures and models prior. To say nothing of how certain dressed would look whilst using the normal movement animation. Perhaps having such cosmetics (and they should be classed as full cosmetics really) causes the avatar to constantly be in "walk" mode too?
J1NG