And it's now alive. The ripple is more than I'd like (16.4 mV p-p) which is out of spec, but close enough I'll probably get away with it for now.
Realistically, I probably shouldn't have used a SMPS for this rail given how much current it actually pulls. But hey, one of the goals of this board was to validate power stuff for future FPGA projects. Negative results are still results.
I need to wrap up soon because tomorrow is a work day but hoping to get some signs of life out of the GTX first.