Simple advertisement on toilet paper

That's the main issue for advertisers:
They want to know that their brands will never be displayed next to anything "immoral" or wrong.
That's why you don't see regular advertising on porn websites for example. brand don't want to be associated with this kind of content.

So would brands really agree to pay knowing that people will literally wipe their asses with their message?

Having worked in an agency, I can tell you that your potential market is really specific and tiny.

Is it possible to buy a toilet paper, print on it and roll it back or does it have to be a part of the production process?