Nice.

My friend posited that the amount in the sewers would be likely to be less than that in the ambient air and therefore worth attempting if desperate.

I don't know much of anything about sewers, but you don't really smell things around manholes, do you? (My anosmia prevents me from answering this with confidence.) If not, it would make sense to presume there's some kind of seal on them. If that's the case, the outlet would be the primary inlet for gas. I don't know enough about gas attacks or drainage to say whether that's a problem.

If I'm ever unable to breathe ambient air or escape the area, I'll try to remember to attempt this and get back to you.

Not all gases. And some gases can be absorbed through the skin, completely invalidating this method. Yay!

Toilets don't get their air from the sewers. There's actually an air hole at the top of the house to let air into the pipes so that they don't produce negative pressure.

Learned that from a plumbing guy who told me that it's actually a noob mistake diy guys often make to forget that hole.