The only ways to get back to the PSP OS, are by either resetting the console or pressing the 'HOME' button. Both of these will lose the game you're playing...so, no, sorry you can't!
And, yes, if you type in a .txt file for a URL it will display the text! I wasn't sure of this myself, so I just uploaded a readme.txt to my webspace and tried to access it...and it worked!
Cheers,
Fox
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