Unfortunately, no. From what I've been told, you can't find external enclosures for 3.5" drives that have auto-sleep features.
The device that the external drive is attached to won't spin down the hard drive. (It's not very smart.) The hard drive will keep whirring away, even when the device is powered off. The only way you can turn the drive off is by killing power to it.
I have the ability to inhibit the 5V line that supplies voltage to the USB-to-SATA controller chip as well as to the HDD itself. I don't know what would happen if I killed 5V and left 12V on the drive. I thought it would be safer to kill the 12V entirely, just like the power switch did.