If I sound like I'm rambling, I am very tired..please bear with me, ok?

My thoughts might be out of order..if I repeat myself, I'm sorry for that too. I have put down a number of thoughts, but the answer you are looking for might be buried in the middle of my post.
Hm....dilated eyes can also be a symptom of interocular pressure...have you had him recently scanned for glaucoma? If not, you may want to confer with Mogs..her cat just went through an eye removal for Glaucoma. Boy, can she ever clue you in as to symptoms. If you've been reading all threads you would have read her 911 thread.
(I frequently forget who I've seen where,on the board. It's called "Old Lady Syndrome"
)
Could be that one of the side effects of phenobarbitol is dilated eyes? Yup. Sometimes medications designed to calm people/animals down, produce effects that seem to be in exact opposition to what they're supposed to fix. This is why some people on antidepressants, commit suicide.
Animals who are even mildly stressed, and have anxiety, may also have dilated eyes.

It's a fear response to allow the animal to see as much area, as clearly as possible. In cats that symptom would be exaggerated since their pupils are slitted. A nervous cat would need to expand his horizontal field of view.
And since Wilbur has seizures, he is certainly aware that something scary happens to him, at times he can't readily predict. This would feel akin to waiting for someone to jump out at him from the bushes at any moment. YIKES!

That would stress ME out! I guess you might call it a form of PTSD.
Epileptics, my ex BIL included..have reported a feeling of anxiety that precedes a seizure. Maybe Wilbur is having petit mal seizures that no one is capable of spotting? They come and go in a fraction of a second, so the only one who would be aware of what is happening, is Wilbur. He doesn't drive, he doesn't do high math,

and he doesn't have an occupation that would need extreme focus all the time. So petit mal seizures could be virtually invisible to us beans

These kinds of seizures could disorient Wilbur..and cause anxiety, AND dilated pupils as he tries to figure out why one second he is on your lap, and the next he's heading for the cat pan, without remembering how he got FROM your lap, into the cat pan. It's akin to driving, and suddenly you find you missed your exit ramp...you get 'spaced out'.
You might have a sensitive kitty, who's very aware of his own body and how it feels, and he might be anticipating seizures. Since he can't talk, he can't tell us if there is something he connects with his seizure state..for example, a slight dizziness, or a ringing in his ears. Something that might hand him a sort of flashback alert.
You mention that you take in strays to rehabilitate..perhaps Wilbur is beginning to have pecking order issues? Are things changing in your home? Anything at all. Is there construction going on outside? New noises, new smells? Do you have a new housemate? Does the housemate have a new girlfriend/boyfriend who comes to visit? Is there another cat who goes in and out of the house, bringing strange smells with it? (Not an outdoor cat...just a cat that gets to go places with it's owner.

). And if you have a strong connection to a shelter, you may not be aware of it, but you are bringing anxiety from other animals home with you, every day..in the form of smells.
Fear has a distinctive odor that can be transferred by clothing. As Wilbur gets older, he might pay more attention to that. Priorities change with age. An older cat might develop an anxiety from feeling like he is 'losing it'..within its home societal structure..Wilbur might anticipate that the social order in the household is changing. Was/is he "Top Cat" in the household? Perhaps he's afraid another cat will take his job?
Add normal emotions and life circumstances and Wilbur could be stressing over just about anything...a very personal thing.
Still..when you see a symptom you don't recognize..it's always wise to pursue general health issues first. THEN worry about the conscious brain.