Using Pyrex casserole dish as bread cloche?

I personally would not recommend it, because there is a risk of cracking or shattering your Pyrex due to sudden temperature changes, particularly at the high heat used for baking bread. If you have a metal (steel) bowl, that would be safer to use.

Pyrex is designed for oven use, so it would work if it's deep enough. You don't even need to have it upside down, I've used my Le Creuset dutch oven as a cloche right side up very successfully, you just take the lid off halfway through. The same would work with pyrex.

The main thing you have to consider is size. Bread will expand considerably, especially if it is a high moisture dough, so anything you use as a cloche needs to be bigger than the dough. I'd say you'd want at least 2 inches (5cm) clearance all around, maybe even more.

If your glass/Pyrex dish is made of Borosilicate glass there shouldbe no problem as this glass handles high temperatures and thermal shock well. Someone already said old Pryex was made of this but no more. Maybe a cost issue or that it scratches easily. No problem for bread making.

I've done it several times with good results. It shapes a round loaf so that it rises up instead of spreading out, and you can see when it has risen enough to bake, but it would be easier to remove the finished bread if it were a real cloche with a knob on top instead of a bowl. I put a lot of oil on the bottom and sides of the bowl so that the bread does slide right out, but still it can be a little tricky getting it out.

The 1.75 qt bowl has the right depth to width for me, but I wish they had a bigger bowl with the same proportions.