Yeah I think this is plain just a no-go. I found the correct plist value to turn it off, but it doesn't work. You can try yourself if you are interested. You will need two bluetooth devices to test to see if its off though, as the bluetooth manager will always be greyed out regardless.
1. Set your mini to discoverable.
2. On your other bluetooth device (other laptop, iphone, whatever), browse for devices. You should see your mini show up in the list.
3. On your dell mini, run these from the command line:
# sudo su -
# defaults write com.apple.Bluetooth ControllerPowerState 0
# killall -HUP blued
4. On your other bluetooth device, browse again, and determine if your mac shows up again.
It should in theory, turn off at that point. My bluetooth adapter I'm using on my hackintosh desktop does in fact turn off when this happens. My mini...not so much.
Just to keep things sane, rerun the same code again with 1 instead of 0 to turn it back on again. Rebooting also seems to do this automatically.