That is exactly how I feel the ipad is. Polished on the outside, secret on the inside
It's not secret on the inside. The hardware and software APIs are extremely well documented. You are confusing your ignorance of the product with an imagined secrecy.
I am not allowed to peek inside nor add any update not approved by the manufacturer
You are allowed to peek inside - Apple even has a free developer program and downloadable tools to let you do exactly that. Download the free OS developer tools, develop any app you like, and install your app on your IOS device.
The only caveat is that Apple won't help you install "whatever update you like". And you most certainly can't sell products on the Apple App Store that don't conform to their rules. But there's nothing stopping *you* from installing *your* apps on *your* devices.
This is what irks me about the supposedly Free Software and Open Source advocates when it comes to Apple's IOS. Free Software could really go to town on IOS. For example, Apple won't distribute MAME through their App Store; and fair enough too. But anybody with a free developer account could compile the source code for MAME for IOS (assuming it exists) into an app, sign that build with their developer certificate, then upload the binary onto their own phone.
Instead people bitch-and-moan that they can't use Apple's App Store to distribute binaries. Why is that a problem? This is a community built on open source and free software. So why not distribute the apps as source. If open-source is such a big deal, why the fascination with bundling everything up as binaries and asking Apple to distribute it?
So you could have any app you like on your iPad or iPhone. The only barrier to entry is you need to know how to compile and install software. Is that really a problem in the Free Software world which has distros like Gentoo? It would keep out all the annoying non-developers too. It would be like the good old days of Linux when everybody actually knew UNIX; before the hoi-polloi found out about it and fucked it up.