I just got off the phone with the guys at YHM and we're good to go on YHM parts. We will standardize on YHM for our custom upper builds. I'm working on finding a supplier for 6.8 SPC bolts, and I'll post as soon as we finalize anything.
So there you have it, we will build custom uppers for our customers who want them. If you would like to use parts other than YHM, let us know which manufacturer and which parts and we'll see what we can do. If you like the great stuff made by YHM, then you are good to go!