Not enough info here, we'd need to hear more about your intended use case before being able to assist more.
- What model + firmware are your Kindles?
- Are you planning on running the devices on battery power during your installation?
- When you say "power-on", do you mean a cold start as in, first boot on a brand new device/completely dead device being charged or just waking the device up as a cover or the power button would do?
- What connectivity options do you have at the site of your installation? If WiFi is an option, it wouldn't be difficult to create a simple server to control brightness over HTTP