The laser is pretty bright so, you would probably be able to mount a sensor to the side of the laser and just have it detect the light that bounces off of the umd. Using a signal from the mobo would be a lot harder not only to figure out what you need to solder to, but also you would risk damaging your motherboard. Also, I would recommend soldering it to the hold switch so that if it does annoy you, you may disable it during a game or whatever.
I have a spare motherboard, so if you would like, I could do the mod on my motherboard, then show a video to prove it functions. Then sell the motherboard to you. (You also could send me your psp, but it isn't smart to do with people you don't know since people on the internet have been known to rip people off.) I am in R&D on Acidmods, so I'm pretty good at working on new hardware mods.
If interested you can im me at:
MSN:
chronicbliss@hotmail.com
AIM: wildog44
Also, if you wanna do it yourself, you can ask me any questions you have.