Awesome case mod! I'm personally not really into case mods, but this strikes me as a sweet project anyway.
For measuring CPU util etc. you might be able to do that by interfacing your tachometer to a serial port and writing a program that gets CPU util from the OS and writes it out on the serial port. Maybe if your tachometer uses voltage levels, you might be able to adjust those with an A to D converter between the serial port & tachometer. I'd try to make a more concrete/helpful suggestion but I don't know the hardware interfaces there.