That's a great idea!

Can you find the nut thing that goes on there? If you could, you could seal it with caulk [use clear or almond, not stark white] and make a rope or macrame holder for it.

How are you planning on supporting the bird bath? Answering that question might help with answering what to use to plug the hole. If it is going to sit on something, i.e. A wood stump, I might plug it with a green, white or black glass disc and some waterproof adhesive. If you plan on using a hollow pole or an object that has some room between the bird bath and the support, I could see a sky blue or royal blue sphere plugging the hole (also with the help of some waterproof adhesive).

You could also plug it from underneath the bird bath using a ceramic tile, a solid pole or a pedestal, once again with some help from a waterproof adhesive.

So, I was considering using the bowl as either a bird bath or a fountain. Was also thinking of perhaps using an iron stand to set it in. I don't have the nut thingee that goes in the opening. A macrame holder is a great idea.

Guess I'm not sure where I could purchase just one round tile the exact size of the opening?

What about a rubber tub stopper? Could I apoxy that to the opening?