Complete list of codes moved to Sega Retro Wiki:
https://segaretro.org/List_of_Dreamcast_Widescreen_Codes
It's a Wiki, so feel free to add additional codes as you make them.
Also, Esppiral wrote a tutorial on how to easily find Widescreen codes using DEmul and Cheat Engine:
http://assemblergames.com/l/threads/widescreen-hacks.58620/page-27#post-900119
Original post:
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These have been done on Gamecube, PS2, and even PS1, so I was wondering why nobody has attempted it on Dreamcast prior to now. I decided to do so, to give me an excuse to play though Shenmue 1 and 2 again. These are codebreaker codes, you still need the enable code for the respective game to use them. Tested on actual hardware, I can post a video if someone wants it for some reason.
Provided in 2 different forms, depending on which you prefer.
Shenmue 1 US: (Updated 2015-10-25)
02230250 43800000 (Original Code, best for VGA)
or
02230250 438E38E4 for equivalent to 720x480 (DVD resolution)
or
02230250 43700000 for equivalent to 854x480 (Actual 16x9)
Shenmue 2 EU: (Updated 2015-10-25)
0231186C 43800000 (Original Code, best for VGA)
or
0231186C 438E38E4 for equivalent to 720x480 (DVD resolution)
or
0231186C 43700000 for equivalent to 854x480 (Actual 16x9)
Before:

After:

Similar to the codes for other platforms, this is anamorphic widescreen so you're not getting higher resolution. Some additional changes could probably be made like shrinking the HUD too, but this is the simplest solution. Consider these version 1. There's still some pop-in occasionally near the edges of the screen, but it's not as bad as when you use an emulator to extend the visible area. That could probably be fixed with deeper analysis into how it decides when something goes out of range.
It's pretty much the equivalent to what happens when you enable widescreen hacks in Devolution when running Gamecube games on Wii U, that also modifies 3D view matrix without messing with the HUDs in various games.
Feel free to port the codes to other versions of the games, these were the versions I own.
Is this something other people are interested in?