AHK: Temporarily turning off a remapped key

Question

I am trying to do something in these lines:

NumpadEnter:: Alt    

!a::    
Hotkey, NumpadEnter, Off    
return    

!a Up::    
Hotkey, NumpadEnter, On    
return   

Essentially, block a remapped alt while a hotkey is pressed down, the hotkey itself being activated through an alt + key combination.

But, the Hotkey command does not work with remappings, of course, so what could be used in its place instead? I am relying already on that NumpadEnter remapping for shift alt and ctrl alt combinations, so simply converting it into a hotkey with send {alt up} and {alt down} is not an option.