Another idea:
Instead of setting hard rank limits on who enters in what round, how about the following:
- Take the list of participants and order by playing strength (I'd suggest using KGS ranks, and using http://senseis.xmp.net/?RankWorldwideComparison for conversions when no KGS rank is available)
- Divide the list into 6 groups of equal size, where group 1 consists of the weakest players, group 2 the next strongest, etc.
- In the first round, group 1 plays group 2. The winners become the winners bracket, the losers the losers bracket.
- In the second round, group 3 enters and plays against the winners bracket. The losers move to the losers bracket, the winners remain in the winners bracket
- In the third round and further rounds, the next groups enter and play against the winners bracket, while the losers bracket also play.
- When all groups have entered, the tournament continues as per normal double knockout rules.
Example:
Suppose there are 48 player, so there are 6 groups of 8 players.
In round 1 groups 1 & 2 play (16 players total).
After round 1, there are 8 players in the winners bracket, and 8 in the losers bracket.
In round 2 group 3 plays against the 8 players in the winners bracket (The losers bracket doesn't play)
After round 2, there are 8 players in the winners bracket, and 16 in the losers bracket (8 from round 1, 8 from round 2)
In round 3 group 4 plays against the 8 players in the winners bracket, while the losers bracket also plays. Losers in the losers bracket are eliminated.
After round 3, there are 8 players in the winners bracket, and 16 in the losers bracket (8 losers from round 3, 8 winners from the round 3 losers bracket)
In round 4 group 5 plays against the 8 players in the winners bracket, while the losers bracket also plays. Losers in the losers bracket are eliminated.
After round 4, there are 8 players in the winners bracket, and 16 in the losers bracket (8 losers from round 4, 8 winners from the round 4 losers bracket)
In round 5 group 6 plays against the 8 players in the winners bracket, while the losers bracket also plays. Losers in the losers bracket are eliminated.
After round 5, there are 8 players in the winners bracket, and 16 in the losers bracket (8 losers from round 5, 8 winners from the round 5 losers bracket)
Now there are no more groups to enter, so the tournament continues as a normal double knockout.
This works pretty well, regardless of group size, as long as the groups are equal size (though group 6 can be smaller without much trouble)