I haven't looked at this game since I gave up a long time ago because of the texture limitation/engine limitations I ran into.
As for how to use a DVORAK layout yes it would be possible. The key map I provided is the hex representation of the ascii table ripped from the US HOTD which should be a keymap for all US-based keyboards -
https://www.ascii-code.com/
Being armed with the hex values for each character and knowing the differences from DVORAK to QWERTY is just a matter of shuffling the hex around as needed.
Perhaps a visual will help:
Here you can see how the two keymaps look - hex on the left, text representation on the right. If I were to flip "A" and "B" around it would make it so in a QWERTY environment when I type "B" "A" would appear on the screen thus changing the keyap.
If you were wanting to do this on the Dreamcast version the same should apply as well. Use the base string of either JP or US depending on which version you're remapping and you can likely just search the entire gdi track in a hex editor for the string to replace.