Sounds like a cool idea if done well.

-Remake the label to list all the NOAC games.
-Screen and controls on back on cart
-Batteries in the bottom part where the pens are

Sounds a bit tight, but it would be a sweet little project.

I say go for it!

Since this is possible, I guess your idea is possible too..

I remember seeing a mod of a NES in a SNES cart, but I can't find the link..

Keep in mind, that mod does not have a lcd which can take up a lot of space.

Yeah I know, but a nes cart is bigger than a controller, so it might be possible.. There are some really small screens out there..

Batteries are a problem though, maybe a thin lithium ion battery, AA's are too thick

You mean something like this:

http://kotomiblog.blogspot.com/2008_01_01_archive.html

(Scroll down to "Fami-card")

I would love to have that!! Have you ever seen any building info on this?

No I mean like

LCD
Controls
Built in games NOAC
Batters (the thin LI ones I guess)

I built exactly what you are asking about.
http://forums.benheck.com/viewtopic.php?t=24946 It actually worked out pretty good

Having a go at doing this too. It is extremely tight.

I would say the toughest bits are finding the thinnest LCD you can and then getting the controls looking right. Battery is not so much an issue. Found a couple of different external lithium packs which will easily fit inside the game pak.

4 posts down http://forums.benheck.com/viewtopic.php?t=24959

Have you seen this?

http://www.instructables.com/id/Nintendo-Portable/