If it’s no go from Noveske, it’s no go from any of their legitimate retailers. The only other way is as you suggested, contact G&R or MSTN to get a custom upper built.
You can always have Noveske Rifleworks build up the rifle or upper you want with the N4 barrel. Know what you want, see if they carry it, and it will be shipped to your door. Trust me.
Grant built an awesome upper for me with an N4 middie barrel. However, I know that he has recently expressed that he is swamped with work. I would check with him first. If he can’t do it, then I would call Neil at Gun Gallery.