It is said that a lot of great inventions in history were done simultaneously in different parts of the world. I guess that applies to us too. I was thinking the exact same thing as you :-)
A few days ago I changed my HTPC from the built-in TVE2 to a single seat TVE3 system and decided that I would like some visual feedback from the system so I can see if it is recording, just like every other VCR/HDD recorder in the world. Using a red LED would be the way to go! If your htpc case has a HDD led you could use that. My case (Dign/Origen H5) does not have a hdd light, but I am sure I would be able to fix a small red led somewhere behind the display window. The actual display is smaller than the window so there should be space. Another option would be to use a 2-colour power led that is blue or green when powered on and red while recording (would require some extra electronics I presume).
Remains the issue of actually controlling a led from MP. For those of you that speak Dutch, I ran across this post:
LEDSdriver – Opname LED voor MCE/VMC
It is a solution for MCE that uses a COM port to switch one or two LEDs. Would a solution like this be viable for MP? I guess a plugin for TVE3 that switches the RTS and/or DTS lines on a COM port depending on the recording state should be possible.