Isso deve funcionar:
SET @prev = NULL;
UPDATE ticks
SET quote= @prev := coalesce(quote, @prev)
ORDER BY tick;
BTW, o mesmo truque funciona para leitura:
SELECT t.tick, @prev := coalesce(t.quote, @prev)
FROM ticks t
JOIN (SELECT @prev:=NULL) as x -- initializes @prev
ORDER BY tick
