Well well well. Rather interesting development.
I got another COAST module from amibay at a fairly high cost. Seller tested the module on a 430fx board prior to shipment, so known good, and theoretically known compatible. Well, it wouldn't boot.
I dunno how it came to me. I was looking at the contacts and it occurred to me that something may be wrong with the connector. I played around with the module when the machine was off. I tried plugging in the module to just after the fingers grab and no further. Lo and behold, the module works.
I tested and there is a definite performance gain vs the async module and it is confirmed to be PB by a DOS utility (so not a case of the module not plugged in enough so the machine boots as if there were nothing there.)
So then I take the original HP module that did not work and tried the same trick. It also worked. However, when i tried seating the module a little deeper in the slot (bottoming out) the computer NEVER boots. The weird thing is, the ASYNC module it came with WILL work when bottomed out completely, but neither of the PB modules will.
So now I am happily running the HP COAST module since it matches the mobo color and will throw the new COAST module on ebay.
CONCLUSION:
If your board won't boot with a PB COAST module installed, back it out of the slot a bit. In my case I see about a half millimeter of the pins when looking at the module from the the side. It is deep enough to be retained securely, but also work.
Bonus:
Small performance bump as expected. From 31 to 36fps in Quake and from 24 to 29 FPS on DN3D rooftop in 640. As the article posted above states, I measured a drop in mem bandwidth with the ASYNC module vs either no or the PB module. But oddly the ASYNC was still faster in games vs no module at all.