The hay is a good way to go when looking for a bridging agent to seal off seepage in a big hole . I would break up the bales though. Add all the bales you can get in the hole and then add a lot of bentonite on top of it all. One way to look at it is to consider the hay as a fiber material. If you ever played pixie sticks and dropped them in a pile on a table, that is what a bridging agent might look like. And then do a very thick cover of bentonite to seal off the small holes in the pixie sticks. If the hole is large enough you might try doing all this in layers.
Wouldn't the hay rot away after a couple of years and allow the hole to open up again? I think geotextile with gravelly "parking lot" clay over it, then bentonite over that might be a better option. Just an opinion.
