I have done a couple BB and I put aluminum foil in the holes then put some plaster like polyfilla on top of it, when dry you just sand a little to make things nice and smooth and voila, you are ready to go! Hope this helps!
Woo hoo, finally I can help someone!

I read to use wood putty. I tried little pebbles to fill most of it and then thinset, but the thinset droops and cracks. I'm having trouble with filling and losing the etched lettering on mine. Arg!

I stuffed the one I taped, with cotton balls, covered with masking tape, covered with duct tape.
it stayed firm.

I don't put anything in mine. I usually try to put a large piece of tess over it and, on some, I just leave them open to prove that it really is a bowling ball! Good luck!
Ann

I fill it with left-over grout. Smooth over the hole...perfect.

becky

I stuff paper towels and then fill up with GE Silicone II. It will set within the hour.

Susie

I only fill in the two finger holes and leave the thumb hole so that it can be put on rebar or something else... if it is not put on anything it can always be put hole down. Also use leftover grout or just mix some up.. cement or thinset works as well.

I only do 2 holes like Emtnest does. I use tightly packed foil and close over that with thinset

What a frugal creative bunch we are!
Thanks ya'll. I'm almost finished with the mirror, and will stuff the little holes and start tomorrow!
Hopefully I will have a camera for some pics.

I have covered them with tess And others have left cuz it's so fun to see peoples faces when you turn it over and show them the BB holes!!!lol!