>covers the user experience
What, exactly do you want to do? What set of programs are part of "the user experience"?

I want it to effectively obstruct the screen and not go away until the user fills a form of a certain length, or resorts to the task manager.

Why not use a screensaver, then?

It is possible to create a giant window the size of the desktop with the title bar sheared off using the Win32 API. However, disabling the ALT-ENTER, CTRL-ESC etc. hotkeys would require you to intercept the keyboard messages, which is not that easy.

Also, OP, what programming language are going to attempt this with? C++, C# or something else?

Just play a Valve game, they're almost impossible to get out of without quitting/crashing/etc.

huehuehuhehuehuehuehuehuehuehuehuehuehue

Inb4 OP releases his stealth.exe

AutoIT is perfect for this purpose. I'm sure there are already screen lockers written and usable for you.

http://tools.suckless.org/slock