The obvious solution is an airplane style bathroom sign. So I made one at Techshop San Francisco, using only the laser cutter.
Step 1: Front Design
Using the picture as a reference, I traced the sink and mirror pattern from the original sign and the Hipmunk mascot, then found a vector version of the Reddit alien and assembled them all together into a 15"x8" rounded rectangle.
Step 2: Case Design
* white border around front to hold next layer
* single-side frosted clear (translucent acrylic causes the pattern to be too blurry), with opaque white border on three sides to hold this layer in place
* white with the pattern cut out
* mirror with the pattern cut out (this layer is not entirely necessary, since it only serves to reduce the light leakage through the layer before it)
* translucent white to diffuse the LED light, I believe it's 60% transmission
* about 8 layers forming the wall of the sign (one of these layers holds captive nuts for the screws that hold the back on)
* mirror acrylic
* white spacer layer
* white layer with wall-mounting holes
it may be possible to sand the sides flat, but I like the rough layered look, so I left it as-is.
The layers are almost all glued together with acrylic glue, except for a gap before the last mirror layer, which is so the back of the sign can be removed.
Step 3: Electronics
* relay: 653-G6RN-1-DC5
* 100 ohm resistors: 293-110-RC
* green LEDs: 941-C503BGCNCY0C0792
* red LEDs: 941-C503BRCNCW0Z0AA2
* copper wire
* magnetic door sensor (self adhesive!)
* stereo cable, 1/4 inch stereo plugs and sockets, and project box from radioshack
* 5V, 2.5A DC power adapter I found in my room
The box contains a relay and a resistor to limit current through the door switch, it then switches the 5V power supply to one of the two conductors in the audio cable. I used the copper shield of the audio cable as a ground and the return path for the current from the sign.
I used a piece of acrylic with lots of holes lasered out to hold all my LEDs (about 11 of each color). The copper wire acts like a bus bar powering the LEDs, one for each color, then a common ground connection.
Step 4: Assembly Test
Note that there is not really enough diffusion with just the top strip of LEDs. I added a strip of frosted acrylic over the strip of LEDs inside the sign to diffuse the light. I also made cutouts of the hipmunk and reddit alien from 60% transparent white acrylic that I placed under the frosted cover layer to further increase the light diffusion.
If I made another sign I would use more LEDs and diffuse the light a lot more.
Step 5: Installation
It works pretty well since people would already leave the bathroom door open when not in use. I got lucky with estimating the wire lengths so I didn't end up having to splice any wires together.
Besides the reportedly amazing productivity benefits, it was a a fun project and, as always, the laser cutters are like making things on ez-mode.