Even if you could implement a simple switch I wouldn't recommend it in an emissions state. IACs are usually a stepper of some sort so a single switch won't work the majority of the time anyways.
The high idle in a manual transmission car is most likely because the manufacturer just took the auto transmission program and flashed it to the manual ECU sans auto trans code.
The reason it is in there for the automatic has to do with transmission safety. Only my Cressida did the high idle when coasting thing and that car couldn't be flat towed. The Tracker would drop to a normal idle at 70 when I put it into neutral and so does the Durango. Both cars are safe to flat tow.
It's likely that if the engine speed isn't high enough in some cars that there isn't enough to keep the bearings pressurized where they need to be. The following is a list of vehicles safe for flat towing. If your car isn't on the list it's likely that you'll have a high idle in neutral.
Cavalier Coupe & Sedan
Alero GX Coupe
Impreza 2.5 Sedan & Wagon
Legacy L & Wagon Versions
WRX & Wagon Versions
Aerio S/GS Sedan & SX Wagon
Celica GT & GT-S
SUVs & Pickups
Blazer 2DR 4WD
Silverado/Sierra 1500 Series 4WD
Suburban/Yukon XL 4WD
Escape 2WD & 4WD
F-250/350 SD 4WD
Ranger 2WD & 4WD
Santa Fe FWD
Grand Cherokee 4WD
B-Series 2WD & 4WD
Tribute DX FWD
Frontier 2WD & 4WD
Pathfinder SE 4WD
Vitara & Grand Vitara 4WD
RAV4 2WD & 4WD
If your car isn't on the list, check the owner's manual. If you have a 2wd version of the 4wd listed check the manual as well. Anything about only towing the drive wheels at a low speed and/or a short distance you probably have a high idle in neutral coasting.
its to keep the synchros in your transmission from wearing out when re-engaging in gear, the closer your rpms are to the speed it will be in gear, the less wear when reengaging, there's a technique to downshift where you rev the rpms to the speed the lower gear will be at when engaging the clutch, it's called rev-matching
my point is, if you understand rev-matching, you can theoretically use a switch, or something, to lower rpms, then undo the switch before you go back into gear
Where the engine is in relation to road speed shouldn't affect the synchros. If you use the clutch when going back into gear the synchros are only used to pull the clutch up to road speed before you use the clutch to bring the engine up to speed.
My little stick shift Metro did the same thing. Found out that the speed sensor was keeping the RPMs high in order to make shifting easier. Once the car came to a complete stop, the idle dropped to factory specs.
I "solved" this by disconnecting the IAC electrical connector. I can now neutral coast at factory idle.