As a Dreamcast owner for years, I looked for some information regarding playing non VGA Dreamcast games in VGA mode as I have an HDTV these days. After some digging online, I found this method. Works most of the time. Some non VGA games will work, others not. After I found this information, hasn't seen it anywhere else as the source I got this (the iso zone) is long gone. The instructions are as follows:
1. Get a Dreamcast CDI image of the game that doesn't support VGA (You can find Online a complete list).
2. Download a Hex Editor software. I downloaded Hexplorer as it is Free. You can use the one you like.
3. Open the CDI image on the Hex Editor. Once it load the code, use the search option to find the following text: SEGAKATANA (Yes. Like you see it. All in Caps and all together).
4. After finding it, you will see the SEGAKATANA text on a separate screen that contains more text info. Slowly check that until you see the letters JUE. Immediately after that, you will see some code that can look like the following examples: 07BBA10 or 01BB800 or 01BBA00.
5. Look for the replace option on the Hex Editor menu. On the space that appear the text or Hex Code to replace, copy the text code that looks like the examples of point 4. On the space that you have to write the replacement text, copy this information: 0799A10
6. Save the CDI image with another name. Better be safe than sorry on this one. Burn this new image on a CD and test it.
That's it! Like I said, works most of the time, but not on every game. I did this method on the following games:
Jet Grind Radio
Nightmare Creatures 2
King of Fighters dream match 99
Of the 5 games I tried, only the last 2 failed to work. You will notice the difference between loading the game using a bypass software like Code Breaker and load the game directly. Hope this will be helpful.