There are plenty of reasons to avoid morphine when other (cleaner) opioids are an option, but I don't think fear of inducing seizures is a good one. Seizures are an extremely rare side effect and I believe have only ever been demonstrated with very high doses (general anesthetic doses) of morphine. Take a look:
The Epilepsy Foundation's Position on Opioids Causing seizures with morphine is probably roughly analogous to causing chest rigidity with fentanyl, in terms of the causes and the risk.
However, I don't see any moral dilemma whatsoever with substituting morphine for fentanyl if the patient wishes to avoid fentanyl, even if fentanyl is an objectively superior drug for most uses, and even if the patient's concerns are unfounded. That's what patient autonomy is all about.
I also don't see any dilemma with just avoiding use of the word "fentanyl". I rarely tell a patient in pain what I'm giving them (unless they ask, of course, but they usually don't), I just say "I've got some IV pain medicine for you, this should work pretty quickly". Also, when I do pre-ops for patients planning to undergo deep sedation or general anesthesia, I never mention the names of any of the multiple drugs the patient is going to receive. So As long as you aren't giving them something that they've explicitly said they don't want, I don't see a problem with giving them "an IV pain medicine" that is routinely used and known to be safe and effective.
I don't remember ever seeing someone express concern over IV fentanyl, but I've heard plenty of people comment on propofol. "Are you gonna use that Michael Jackson drug on me during my colonoscopy?" is something I've heard many times. Sometimes they are just trying to be funny, but sometimes they are legitimately concerned. I always say something like "Yep. It's an excellent drug and it is very safe when you are having it given by and being closely monitored by a properly trained clinician. Unfortunately, that wasn't the case for Michael Jackson, but we're going to take much better care of you than his doctor did him". Maybe you could come up with a similar line to say when people express concern over fentanyl.