Marine Design Corp makes the IAC survival harness kits. These will fit the 496 also.

From their website:The Marine Design Corporation IAC Survival Kit is designed specifically for Mercury Racing 600SCi, 662SCi and 700SCi engines. Our kit eliminates costly PCM damage that can occur when an IAC fails, which can be a common occurrence. The kit was developed for twin engine applications and includes two plug-and-play IAC harnesses that are fused to prevent a direct short to the PCM. Also included are two Tier 1 OEM IAC units with gaskets, spare fuses and a canvas storage pouch. Replacement IACs and fuses are also available.

The IAC Harnesses only kit will fit the above engines as well as the 496MAG, 496MAG HO, 525EFI, 850SCi, 1075/1200SCi

So whatever failure mode the listed engines have other Merc MPI models will not have a risk of the ECM failure due to IAC? I wonder what specifically the difference is that would cause the IAC to, presumably, draw too much current and damage the ECM.

I'm not certain on that, but I do know the engines that run the PCM555 Motorola setup are very prone to damage from a failing IAC valve.