I need to grease a baking pan (which I don't have) for brownies. I also don't have butter or shortening. I do have one of those pyrex oven safe glass pans and something like a small dutch oven, but with a glass top instead. Could either of these substitute for baking pan instead? And in lieu of the butter, could I use vegetable oil and a paper towel to grease one of the pans that I do have (without affecting the final results of the brownies)?
Vegetable oil will be fine - that's what's in commercial nonstick spray oils anyway.
A pyrex oven safe glass pan is a baking pan. The baking time might be slightly different than with a metal pan, but it will work perfectly fine. Just make sure it's the same size pan the recipe called for, or scale the recipe to match the area - if you change the thickness, the results are going to change.
The dutch oven (uncovered) would also work, though it will take a lot longer to heat up, so it will probably affect the baking time substantially, and you may have to be careful not to overcook the top.
If you are particularly meticulous, you should be able to do it by lining the pan (or at the very least the bottom) with parchment paper...
Even if you line the bottom w/ parchment paper, you can use a knife to loosen the edges
Either of your pans should work - the issue that you're going to have is adjusting the amount of batter to account for a different pan size (possibly).