Had the same issue with a 2009 iMac. Many months later I finally got back to investigating this. I removed the LCD panel again, carefully disconnecting everything. With the LCD off and the RAM removed, I powered-on and everything fires up and you get the constant beeping (which I believe is the 'no-ram' notification). I installed the ram and tried again. It turned on once more! At this point, with a friend's assistance, I held the LCD in place and connected one cable at a time -- then powering up.
The result was that the LCD powered back on before I put the V-sync cable in (the last cable you usually connect.).
So the result for me was that the Vertical sync cable was damaged which caused the no power, no fans, no nothing, issue. That makes no sense to me but I put the iMac back together without the V-sync cable attached and everything seems to be running fine. Since I don't do any graphic-intense activities on this iMac, I think I'll just leave it disconnected.
Hope this helps others in the same boat.