Using 16 hole front rim to build a rear wheel
Sixteen spoke rear wheels do exist, but there aren't very many of them, and to function they need to have extremely stiff and strong rims. In this case, being a Dura-Ace product, it's reasonable to assume that the everything has already been pushed to around the limit in the design. In particular, making one of these a rear would require the compromise of either pushing the drive side spoke holes way beyond what they were expected to endure in terms of resisting cracking, or making the whole thing have way less tension than intended, resulting in it being weak. Just winging it and assuming you'll be fine is not a good idea here.

These limitations don't have to do with the rim being carbon per se.

Worth mentioning but pretty far down the list of issues with this plan is that the sort of rims these are (deep carbon rims in an ultra-premium prefab set) can have directional drilling that will contort the nipples around if a pattern other than intended is used.