Unfortunately, it's probably going to have a lot of seeds in it...

It is and weed seeds should not be a problem now after it has been sitting around for 6 months. Any weed seeds in the bale should have germinated and died by this time.

I use old hay for all my veggie garden mulching - makes a great mulch! Kimmsr is right - all the weed seeds should have germinated by now. You'll find that it makes a great mulch, and adds organic matter to your beds as it decays!
Kathy

Yep,
I use hay (old and new) as a thick mulch on everything all the time and have had success since 1972. See if you can find any of Ruth Stout's books on permanent mulch.
Pat

I use grass hay becausen I don't need to seed a cover crop at the end of the year.

And like Ruth Stout said-"if you see a weed, you dont have enough hay on it".