Some further thoughts & info for those who might have decided to check this thread out....
As to when, I would suggest sticking with the May Day bank holiday. Which, just to be confusing, in 2020 is a Friday, May 8 (for the VE anniversary). Which would make this the first FFNRtK. Those who wish/need to have a rest before travelling home would then still have the Sunday for doing so. Alternatively, the second holiday weekend, so Saturday 23 May, which leaves Sunday & Monday for travel and recovery.
The halfway stop would be
t'Hoekske in Ghent. This is a 24 hour snack bar, which by all accounts does a most decent service. The only alternative would be vending machines or self-catered options, as there is literally nothing else open in Flanders overnight. We've generally stopped in Bruges for a further break, and then on to a late breakfast at
Patisserie Caruso in Ostend, generally arriving there at 10 or so.
Getting there:
1. Megabus is a cheap option, if you have a folder. it would be about fourteen hours for me, door to to door.....
2. Ferry from Dover to Dunkerque, ride across the border, either take the coastal tram or ride on to De Panne, train(s) into Brussels- either direct or changing at Gent St Pieters for a faster service. Belgian railways require a separate bike ticket (€4 per journey)- no reservations, so it's subject to space being available. A dedicated bike space seems hit and miss in practice- some trains have a huge dedicated space, others I've ended up in the vestibule. Folders travel for free if folded.
3. Eurostar. By a country mile, the easiest option, particularly from further afield. Full-size bikes need to be booked (separately from train booking, unfortunately) with their luggage service, EuroDespatch. There are usually two spaces per train for fully assembled bikes (now £35 when booked more than 120 days in advance), boxed bikes cost £30 (they can supply their own). Best option is to ring ED (no online booking for luggage) and have the booking page at Eurostar.com open at the same time so you can match availability/pricing to suit. You can also get the bike on an earlier/later train and pick it up from the Brussels South luggage office. Folders travel free, they must be no larger than 85cm in any one direction and bagged before you go through the security gates. They're not too precise about the sizing- Chutney my Dahon is ever so slightly larger folded, but there was no problem getting over and back.