I know that there is a possibility that models from the Dreamcast version of Sonic Adventure could be extracted, but I'm not sure it has been attempted yet.
Most of the textures in Sonic Adventure reside in .PRS & .PVR files. The majority of the textures reside in the .PRS files, which are compressed .PVM files, and which are also compressed .PVR files. Unfortunately I have not been able to extract any models from the .MLT files.
Here are some texture hacks I have done myself with the Dreamcast version:
http://forums.segakatana.com/viewtopic.php?t=85
Here is a thread on Sonic Adventure models, but this was done with the PC version:
http://forums.sonic-cult.org/index.php?showtopic=941
I hope that helps some!
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