If you think for MAME4ALL, I suppose that is is not possible, because MAME4ALL is based on very old MAME.
Anyway I never try to develop something with network for Dreamcast.
The question is: Haw many people uses modem today? and, How many people have a BBA? I think that it is not a lot of people.
But maybe I could develop network play for some multiplatform game at far future and play with people that uses other platforms.