A way to handle the situation, Etops. Since you said you might mention it to the crew during the briefing, I'm assuming you are a pilot.
Before the briefing, ask the #1/A/Lead flight attendant about it this way...
"I was non-revving on another airline, we diverted, safety demo done again before take off, no one got on or off, yada, yada, yada. I asked one of their f/as why safety demo repeated, and he/she/it said that it was an FAR and it had to be done. I've never heard this before. Could you look it up in your manual, and show me what our policy is?"
If the flight attendant says, "Oh it's an FAR, but we don't bother doing it if no one gets on or off," then you have a problem that someone in Procedures/Safety/ Compliance needs to know about. They can send out a broadcast message to all f/as reminding them of the FAR.
At AA we just got one the other day regarding mininum crew staying on the a/c until all passengers have deplaned. You would think that this would not have to be repeated. Yet, the very next day I worked a flight where the #1 got off the plane to escort an unaccompanied minor seated in F/C up to the gate. No one in coach had deplaned yet. Our written policy is that UMs have to wait on the a/c until all other passengers have deplaned to comply with the minimum crew FAR. It has been the policy throughout my vast career of 5 years, and the #1 was senior to me; so, I know it's not because she had never heard this before.
If the manual does not mention it, then you still have a problem because it is an FAR and someone in the company needs to know before the company is fined. The fines start at $25,000 per violation.
A friend of mine at Frontier brought a similar F/A manual omission to the company's attention. (Turned out that the FAR had been addressed in previous editions of the manual, but had been inadvertantly deleted in a revision.) The company was grateful for the heads up. I would doubt that JetBlue would be any less grateful. FAA fines for this kind of thing are the most useless expense an airline has to endure. There's just no reason for the fines other than deliberate violation on the part of employees.