Your dilemma is often referred to as "The Holy Grail" of modding, which is salvaging an embedded lcd monitor, like one from an imac, or laptop to use elsewhere. The problem lies within the lcd controller being embedded within the mobo of the product it was installed on ( http://www.applefritter.com/node/7135 ) and making it a cost issue.
I agree with Olelmuvun, and suggest using a product like this...
http://www.sonnettech.com/product/maccuffmini.html
or this....
http://www.h-sq.com/products/minimount/index.html

but jamming it inside the G5 case with a similar enough lcd would be f@*king rad and would require full documentation and photos to impress and inspire nerds like myself to make some cool mods!!!!