OK, so I knew this was true. But there is plenty of confusion about whether Pro Tools HD PCI cards are PCI or PCI-X. They have the PCI-X form factor and the recent PE6R4-I chassis is a PCI-X chassis.
So I took my chassis and Macbook Pro into work and loaded it with an HD2 configuration. The cards hang over the end of the PCI connector on the chassis' motherboard. But that's OK.
Since this is my eventual plan (when I can afford to buy PCI cards and at least one audio interface second-hand), I'm happy to report that it works! The weird thing is the disappearance of menu bar items on the right side—on my Macbook, that's the Time Machine, Bluetooth, Volume, Battery, and Date/Time icons. The Spotlight icon does not disappear.
But HD is still functional. And the menu bar thing is a known issue.