Here is what actually happened!
I got to Lille Europe early enough to honour the one hour requirement and checked with a member of the Eurostar staff who said I didn't need to take this into account; I was therefore able to kill a bit more time with family.
I returned to the station fifteen minutes before departure, by which time the train's platform had been announced, and sure enough it was a non-secure platform. I was able to go onto the platform with my family without any checks, neither for ticket nor for security.
While we waited for the train to arrive, a security guard came to confirm that we were indeed travelling to Brussels. I explained that it was just me, and he asked to see my ticket. The fact that my wife and children were also on the platform didn't pose a problem and they were able to stay and wait for the train to depart.
So, it was as I suspected, the ticket for Lille>Brussels is exactly the same format as the other tickets, including the warning about being an hour early, but in the end it wasn't necessary.
I was put in carriage 8, which is in the middle of the train.
Please also see Ben Watson's post.
One other thing: apart from a visual inspection of my ticket by the security guard (he didn't scan it) there was no other check of my ticket on the whole journey: not on the train, nor on my arrival at Brussels.