Hijack does something very similar to your wish.

If you turn on the screen blanker feature, then after a playlist completes, and the player is idle, the blanker begins an immediate countdown, regardless of screen activity. After it eventually blanks the screen, a standby timer starts counting down. When the standby timer expires, the empeg is placed in to standby condition (yellow led blinking, display powered off).

The standby timer defaults to 30mins (I think), but can be changed in config.ini using:

[hijack]
standby_minutes=5 ; change standby timer to 5 minutes after blanker

Cheers