I'm not intimately familiar with 2k/2k3's event commands, but are there event commands for either Transparency ON, Set Player Opacity, or Change Actor Graphic?
If you can find those...
Transparency ON: will make the player invisible (set transparency OFF to end that)
Set Player Opacity: set to 0 to make player invisible; set to 255 to show them at full opacity again
Change Actor Graphic: this is how you can change which actor is shown as the player; also, you could set to an empty graphic file to make the player invisible