Presumably a large VU meter then... Sounds a reasonable implementation - find out how long the servo takes to respond to a seek and update at that sort of rate. Because it'll be always active you'll need to make sure the power supply is up to the task. I don't know what the lifetime of a servo is if it's constantly moving - might be an issue?
Another possible idea is using the head-positioning mechanism from an old dead disc drive and adapting it - Voice coil in hard drive, as a speaker for - Faster Harder Scooter - YouTube