Wow, that's one amazing piece of custom work!!! If you are totally opposed to wall mounting, then yes, a custom bracket is the way to go. Looks like you are already into this deep enough, and you definitely possess the skills to fabricate a custom mount and make it look perfect it would seem.
I get it, after so much work on this project of yours, with the finish line in sight, the last thing you want to do is deepen the scope. Given the money you would save on the 900F and the logic of putting that towards audio makes complete sense to me. As a musician, my money definitely favors audio quality too. My 85" arrives tomorrow so if you need any specific info PM me. Otherwise, good luck on the credenza ...post some pics once it is done and setup with the new 900F custom mounted on top! :-)
Main: Sony X900F 85" LED, Denon AVRX7200WA, Klipsh RC64II, RF7II (x2), CDT5650 C II (x8), PSA 15V Sub, RSL Speedwoofer 10S Sub
Home Gym: AVR3312CI, RC62II , RF62 II (x2), Polk RC80i (x2), R112SW Sub, Panasonic 65" Plasma
Zone #2 (Lanai): Denon HEOS Amp, Polk RC80i (x2), LG 43" LED DJ Studio: Event PS8's