Has anyone seen anything that would fit the bill?
Inphinity: I don't think you'll find anything close to the NUC form factor with a full PCI-E slot, simply because of the impracticality of fitting the majority of PCI-e cards into the casing. Even Lenovo's Ultra Tiny only has mPCIe.
What do you need the PCIe slot for? perhaps there's another way to achieve your requirements.
Failing that, Asus' ET series of AiOs is great.
Dairyxox: HP Compaq Elitebook is fairly small (260x60x260mm) and has an optional MXM module for discreet (laptop style) graphics.
Its not NUC sized, but small, powerfull, quiet, and can have discreet graphics.