This explanation is from my fuzzy memory (which may not be correct) of the 2nd edition illusionist (which might not be what you want).
A lot of the illusionist spells would boil down to simple game mechanics (that may or may not be active at this time). For example, most of the illusionist spells would be a morale check, like audible glamor or phantasmal force. And the more powerful spells could grant a penalty to the check. Some of the higher level spells that make "quatsi-real" creatures would be like summoning spells.
Now, to use this against the player(s) outside of combat would require more creativity on the mod designer; but it would be like some of the strange stuff they put in the Goldbox games. An example of this would be the black circle mage who casts a fireball at you and you dispel it. I still haven't figured out if the GPDL scripts let you remove a spell from a characters spell book or not, but I haven't tried DC much lately.
ST