When changing my hotkey layout, there is no way to remap the Mehter’s abilities, even in Fully Remappable Mode. It will always just use the grid keys. This seems to be because the abilities aren’t defined in the .rkp hotkey files nor do they even exist in “C:\Program Files (x86)\Steam\steamapps\common\Age of Empires IV\cardinal\attrib\instances\hotkeys\hotkey_binding\abilities”
(This is potentially related to the identical problem with Malian workers abilities I made a separate report about)