For my extended 2018 banked points that expire at the end of Nov 2020, I can now see them online as Riviera points.
I have tried for the last few days to book online specifying to use the "Riviera" points first. I get all the way to the last page (the Review your booking page where you have to check the box to accept the terms and conditions), but when check that box and click "Book this room", I get the Dwarfs We're working on it page rather than the congratulations your vacation is confirmed page.
However, if I specify to use our regular points first, and the reservation does not require any Riviera points the reservation goes through, and I get the congratulations your room is confirmed. I then can call MS to have the points reallocated to Riviera points.
So from my experience, even though the Riviera points show up online, you cannot book online using those points.
ETA: See @Sandisw reply below and my confirmation in reply to her post for a work around