An update - I worked out how to add the extra "folding bike luggage" for €5 online, so decided that I would prefer to shell out that much and be 100% confident I was legal rather than run the risk of trying to explain in bad French why I hadn't!
The boarding process was pretty scrum-like - around 1000 people to get on a train takes some doing (more than Eurostar) so no wonder boarding in Lille started 20 mins before departure. I am really not sure that anyone official took much notice of what luggage anyone had, but the extra luggae purchase would no doubt have been visible to the person scanning my QR code to let me onto the platform.
Dragging the half folded bike down the platform in the heat, given that I was in the second to the front carriage of the double decker train and the trains are around 400m long, was not fun, but finding a space to stow it was not an issue (unlike the 'classic' TGV I came back on where luggage space seemed in rather short supply).
Was it worth it? Yes - I had a quick ride around Lille between trains and then later in the day an enjoyable 35kms around Lyon, plus an easy way to get home when I returned to St Pancras.
