Replacing object characters with graphics?

[meldels]

Is it possible to create custom graphics for ZZT to use? (possibly a font?)

[Saxxon]

You can create a font for it- Font Mania is a popular font creation program that will generate a COM file that can be run in DOS to apply it. A few games work like this.

[Schroedingers Cat]

It's worth pointing out that not everyone will use your custom font, so design whatever games use it with that in mind (unless you're working on the font alone).

[meldels]

Is it something that can be defined in-game, so I could just package it with the .zzt?

[Commodore]

Not really. You'll have three files:

game.zzt
font.com
start.bat

The bat file, runs the font file, then runs zzt. Just search the archive for "font" and look at the games. You can edit the .bat files in notepad or similar.