Is the FZ120 compatible with the BTF80/90 LGA775 back plate, so that the latter can be replaced with the former?
I have the BTF90 and the mounting hardware looks identical so I would think so. You'd have to hope the back plate doesn't "escape" down the back of the motherboard or you'd have to have the motherboard out anyway!
Next time you test a heat pipe based cooler, try orientate the motherboard differently to see how it affects heat pipe performance. This might be a weak spot for heat pipes, as they partly depend on gravity for their operation. Whats the performance gain/loss between mounting it in a tower vs. desktop chassis?
If you take a look at page 4 of the XP-120 review
you'll find that SPCR did just this test. The conclusion was that it made no difference, the "wick" in the heatpipe(s) brings the fluid back to the hot end fast enough. I doubt any modern coolers use heatpipes that would have a problem with any orientation.