HeavySpleef includes a function to add a realtime scoreboard to a 1vs1 game. The syntax for the command is the following: /spleef addscoreboard <arena>. If you look in the right direction, there will now be a scoreboard left-under you. This scoreboard changes its numbers every time a player won a round. This perfectly for a spleef tournament.
Also, scoreboard blocktypes can be configured in the config.yml. You find a config value for the numberid and for the blocks around the numbers (otherId).l
Please note that this feature does only work on 1vs1 games!
A scoreboard may look like this: