I don't know if you can do that with autofs, but maybe it's possible to call an external script when the drive is auto-unmounted. This script could change the LED to report the drive mount status.
Or if you're not in a hurry you could run that script from a cronjob every minute (not great but cheaper than a hardware solution)
Another possibility: what about plugging in a cheap USB mouse (you'll need a USB hub too of course) and map one of the buttons to call the unmount+LED check script? I have no idea how you can remap mouse buttons to call a program without being logged in but it's probably possible without too much hacking...
I already use autofs for automounting.
Besides I find the unmount feature in autofs pretty insecure. The manual says, the HDD gets unmounted when itīs not used. So before I plug off the USB-HDD I must know if it is already in mounted or unmounted state;
Now that is a point I didn't consider: I must have something like a feedback from the plug if the unmount procedure has succeeded (sometimes it is not possible to unmount a HDD). I think flashing one of the LEDs would be the right thing...
So it should look like: press button to unmount; wait some secs; if the blue LED is blinking longer than 5 seconds the unmount procedure succeeded; the USB-HDD can be now removed from the USB port.