Gavin did the bulk of the 1 character aspect, but I basically know what he did (And I had to finish the rest of it).

General  Hacks:

The Character Select Screen was disabled by making it jump directly to the Bridge Crossing event; however, the code for the screen was gutted to allow custom code done by Gavin and I to be run immediately before this.

Select was nullified to prevent access to the Lineup screen.

The character classes were manually set up by loading their values to their respective SRAM addresses.

The Main Menu was hacked to remove the player sprites from being displayed (Sargon's portrait is actually the Orb tiles). Also the Magic menu was hacked to immediatly load Player 1's menu on select, and Status was removed (It is displayed on the Main Menu itself).

Buying weapons/armor/spells in shops was hacked to immediately load them into Player 1's slot with no choice of who to take it. Selling was also hacked to load only Player 1's inventory instead of asking who wanted to sell something.

The dead players had their inventories stuffed with invisible dummy items: This prevented finding an item in a chest from being placed in their inventories.

The weapon/armor menu was hacked to disallow selection of the dead player's slots.

Battle Related Hacks:

The other three players are set to "Dead" on the first battle of the game. Sargon's battle sprite is actually composed of their corpses, and Player 1 is invisible except for the weapon animation (For the muzzle flash).
The X/Y coordinates of them were also changed to make the composite Sargon sprite. The Clinics were also removed from the game to prevent any of them from being resurrected.

Using a HEAL/Etc was hacked to immediately choose Player 1 as the target.

There may be a few things I'm forgetting, but it wasn't exactly simple to pull off: It was probably the biggest programming task of the hack, though there are many more, such as the expanded spell names, new item effects, the screen fade on entering/exiting towns being disabled, the color cycle on Shop entering being removed, etc.

You'll notice DXOII starts you with two PSIs and an (Equipped) armor set.

Digging through some of my notes...

Weapon Slots
------------
6158, 6159, 615A, 615B: Player 2

6198, 6199, 619A, 619B: Player 3

61D8, 61D9, 61DA, 61DB: Player 4



Armor Slots
-----------
615C, 615D, 615E, 615F: Player 2

619C, 619D, 619E, 619F: Player 3

61DC, 61DD, 61DE, 61DF: Player 4

Subtract #$40 to get Player 1's slots.

Not sure how useful all of this is, but some of it will be.

$39970 - Code to load starting items/configure player sprites. (This is in the area the Character Select Screen occupied)

$399A9: A90F8D2261 Load Master DEF (This was needed to set DEF to the combined values I'd assigned for the two armors you start with: If this isn't done, Player 1 will have a DEF of 0 until you check the armor screen).

$3F311 : X coordinate Player 1?

4C43C1 Jump to Intro (This is the jump to the Bridge Crossing sequence).

$6300: PSI Slot 1

$6301: PSI Slot 2

Extrapolate as needed.