I'm not sure how complex modern display controllers are, but I could imagine a simple priority hardware overlay functionality that an exclave has access to (similar to the dedicated "cursor overlay" functionality some older GPUs had, as far as I understand).
Once you have that, you can take the idea further: Displaying an indicator that confirms that all your keystrokes are going to an exclave validating your password, for example.
The much-hated touch bar actually enabled just that, for Apple Pay payments, as far as I remember: It could display something like "touch to confirm payment of $x" on its own screen in a way that was impossible to manipulate from macOS – now here's an opportunity to bring that level of security back without requiring a dedicated display or taking away people's beloved function keys.
Once you have that, you can take the idea further: Displaying an indicator that confirms that all your keystrokes are going to an exclave validating your password, for example.
The much-hated touch bar actually enabled just that, for Apple Pay payments, as far as I remember: It could display something like "touch to confirm payment of $x" on its own screen in a way that was impossible to manipulate from macOS – now here's an opportunity to bring that level of security back without requiring a dedicated display or taking away people's beloved function keys.