Hi, I've been experimenting a bit and found out that none of the Mac's OPTION keys are not registered by WinUAE. At all. While it is possible to remap keys in WinUAE, Mac's OPTION keys cannot be used because WinUAE simply doesn't see them.
What you can do is to use some other Mac's key as a LEFT or RIGHT AMIGA key, for example the one left from key 1. In WinUAE it's called "GRAVE" (for whatever reason
Launch WinUAE (by pressing "Configure" button in AmiKit Launcher), go to "Input" settings, switch to "Configuration #1", find this "GRAVE" entry and change its "Input target" from "Back Quote" to eg. "Left Amiga". Similarly you can assign "Right Amiga" key.
Don't forget to save your new WinUAE config. Go to Configurations, mark "AmiKit (Default Configuration)" and then press SAVE button.
I know this remapping is not the most convenient way, but at least it works.