Forgive the n00b question, but I only found out about itch.io this week.

If I use what I know best (a 640x480 pixel div, lots of jquery), is that going to work okay with itch, or will I need a 'real' engine like RPG Maker to be able to submit?

That sounds like it would be perfectly fine! Twines are popular, for example, and the game files are just HTML pages.

That will work just fine. All my games on itch.io are html5, although I haven't used jquery in years :)

Just make sure your main page is called index.html, zip it up with all your js code and you should be fine.

This is excellent news. Thanks everyone!