I haven't needed to use the online booking as yet and just had a look...hmmm.
We used to do Teddington to Limehouse in one but because of the 4 hour tidal window for Limehouse lock, we now split the trip over two days and stop overnight at Chiswick pier. The overnight stop allows us to have the ebb tide working fully for us both times.
I would still call the lock office (I presume that is still an option?) a couple of days before to confirm they have my booking, confirm my arrival time and see if they have a place on the wall for me, although i think that is becoming less of an option. Staying on the wall for a few nights on our barge was a cheap alternative compared to the marina moorings or indeed other marinas.
I will follow my passage plan according to the tides and timings for my barge regardless of what the bookings computer says, if that is an issue i won't bother...shame, as it is a great trip and we liked Limehouse.
Let us know how you get on Mr Magnet if you are the first to use the new system.