I guess 400 participants is the ideal headcount for the PAPA facillity as it is now. As Mark points out. Atleast for the main event of two days qualifing.
The only way to raise the number of players able to compete for the Pinburgh trophy, as I see it, is to make a runner up, pre-qualify, tournament on the Thursday. With no participants limit. The top x players of this tournament (say 20) will be able to fill up x reserved spots in the main tournament.
I'm not saying it is easy - but it's doable.
What is not so easy is to determain who may sign up directly, the number of reserved spots, and tournament format.