I'll try to help you, since I am not too much familiar with the eMMC drivers in Coreboot. I bet there are none, but there must be BCT support as you outlined it: "Enable PCI mode for SCC devices" (PcdSccEnablePciMode)".
I never tried to set eMMC on BayTrail on Coreboot (in fact, I never tried BayTrail with Coreboot, but I did try others' companies Coreboot images on once upon a time BayleyBay Fab.3 E3826 (dual Core) CRB). But I did try all of (and more) what you are writing here with various INTEL and AMI BIOSes. It is off topic.
Here is what I did try on some other customized platform, using E3845 B3 and D0 steppings, and AMI BIOS. This is how I did setup with South Bridge, SCC controller:
Indeed, I was able to install Linux on eMMC (it was eMMC 5.0 compliant, 32GB size, paired with BYT-I as written above, which supports up to eMMC 4.51), namely Fedora 21 (if I recall correctly), and it worked seamlessly.
I admit, I never tried WIN, but I do know that there are special INTEL drivers developed for embedded WIN (for WEC 7 and WEC 2013). I am not sure if such amenities exist for WES 7 (indirectly for WIN 7), you should check these yourself: http://www.intel.com/content/www/us/en/embedded/products/bay-trail/software-and-drivers.html
Maybe you can find some WIN eMMC driver support on this site. At least I hope. Hope dies last. ;-)
Hope this helps/nadejus6 eto pomozet,
Zoran