Well it's more centrally acting than say clonidine, given tizanidine's higher selectivity for the alpha -2C receptors, so it's going to have more central effects. It's also more sedating from what I recall. My guess is that any hallucinations occur between sleeping and waking states, especially from the sleep state to waking state (hypnopomic hallucinations). But I'm sure it could occur in either direction. I don't think the hallucinations would resemble those produced by z-drugs, which occur during the waking state and are more overt. For instance, take 60mg of zolipidem and fight to stay away and just about anyone will start hallucinating. I guess the only commonality would be a dream-like quality.
Hypnopompic hallucinations are weird. I remember waking up at like 4am, and in the shadows of the room (the room had some light trickling in from the blinds) I saw a homeless black dude leaning against my wall, standing with one leg on the ground and the other bent, touching the wall. I never really saw him closely or talked to him, but my body was completely paralyzed -- I couldn't move a muscle. The paralysis was far more scary than the black dude, but he definitely was ominous. It felt like it lasted 15 minutes, but who knows how long it was.
My guess is that tizanidine hallucinations are of that nature; as you are waking up the typical noradrenergic system that helps you "turn on" isn't functioning fully, letting your mind go to transient places for atypical periods of time, such as waking dreams states.
