Can I save a too-reduced basalmic reduction?

It sounds a little weird but what you have is awesome on ice cream and strawberrys! Try it.

As almost everyone else has said - adding a little water should help! However, depending on how reduced it is, you may have something great! I once worked in a restaurant where we made a very thick almost caramely balsamic reduction and drizzled it around and over a wonderful portobello-chevre-spinach-roasted pepper Napoleon. Yum. I'm drooling just thinking about it!

Add some caramelized onions and a pinch of brown sugar to make a sweet onion jam

Well, not exactly. You only make the mistake of putting your head over the saucier once.

All that reducing does is remove water, so I would think you could add water, as hardlikearmour says.

Yep, water should do the trick.

I vote thin it out a bit with a little water.