I agree, a G4 clamshell is neither realistically possible, nor a practical project to take on.

As for Tiger on clamshells, I'm not an expert at machines that old, but a brief search on the web seems to indicate that Panther is the highest they'll go without resorting to extreme measures:

http://forums.macosxhints.com/archive/in...

If you do try to go the Tiger route, you'll need to find the Tiger 4 CD set, or have an external optical/hard drive to use to host the software. It's just not an OS that was around during the time clamshells were prevalent, so getting one loaded with it is not going to be a straightforward upgrade.