This is exactly what I want to do as well: bridge dialup to ethernet using a pi. Did this work? Does anyone have any further feedback about the possibility of this working?
The dreamcast needs to hear a dial tone and dial out in order to make a connection. I've seen where this would require a voltage or something.
I remember reading about a "dreampi" project, but that involves running a voltage throughout the phone lines in the entire house. I was hoping to avoid that step, and just have a single bridge in a single box.
I've also found this: https://www.howtoforge.com/linux_dialin_server
I'd like to know if anyone has already made this work, I don't feel qualified to pioneer this project myself, ha.