There is no way to play Freedoom directly on the Dreamcast, because it requires a 'boom' compatible port. Not only Freedoom, but a few player made mods as well, for instance Deus Vult I - II.
Currently a modified version of PrBoom is supported by RockBox (an alternative firmware for portable media players)
, and a modified version of PrBoom was used for the iPhone / iPod Touch 'Doom Classic'
I know I might take a little flak for asking this, and without asking if someone would consider doing this, could someone tell me if it's technically feasible to port either one of these to the DC?