Have a persistent "developer mode" flag.
When the device is being set up for the first time, ask the user if they want to enable developer mode. Make the warnings as scary as you like. When the device is booting, display a prominent "developer mode enabled" message. But, once the device is booted, there should be no way for apps to query developer-mode status, to prevent discriminatory apps.
The only way to toggle the flag after setup would entail a full factory reset of the device. You could go one step further and have it be a fully permanent flag, in efuses.
replies(1):