Can I buy an ideal diode? How do I OR two power supplies together
That's right, your reverse protection chip uses an internal MOSFET to achieve lossless switching.

I seem to recall seeing a "drop-in replacement for a diode" kind of FET-based device, but I can't seem to find it now.

MOSFET's with dedicated secondary-side controllers are used for synchronous rectification in higher-end modern power supplies (and in low-volt VRM's on modern PC motherboards).

Speaking of a "diode OR" in a redundant power supply scenario, I've found another relevant link at LT's website...