Could I get away with using this?

Even though Pearl Jam performed this, they never copyrighted it nor did they ever release it for profit. Would a wad that used it still get knocked off /idgames for copyright infringement? (this question also applies to their Small Mosquito demo, which I was also considering using)

In US copyright law, a work is copyrighted the instant that it exists in a fixed form. The formality of registration is important for granting certain legal rights that make pursuing infringement more feasible, but the song is already copyrighted as soon as it's been recorded or written down, regardless of whether it was ever registered officially.

For what it's worth, I believe there are some projects with illegally-used MP3 music that have managed to sneak onto idgames in spite of the rules against it, but that's neither here nor there.

esselfortium said:

In US copyright law, a work is copyrighted the instant that it exists in a fixed form.

Actually one of the basic tenets of international copyright laws, not just US.