Introducing the Gauntlet
You can create a Gauntlet-type Bracket on Toornament, to have a Playoffs stage that gives a great advantage to the participants that finished the previous stage of your tournament with the best seeds.
This stage is indeed a Single-Elimination Bracket in which there is a single match per round, and participants are seeded up to the final match from the get go:
A single match is played between the two lowest seeds of the stage, and the winner advances against the next seed etc.
Here is an example, in its dedicated widget, for the League of Legends LCK Summer Playoffs:
Where the ranking at the end of the regular season was: