I've had some experience setting up qualifiers for main events. It's hard. I think they depend on players who simply want to enter a competition with no intent or plan to play in the Open. I think they will need at least 50 events to fill 128 spots for the US Open.pretty sure they will fill. figuring in a couple asia qualifiers and euro qualifiers, added to the many US based qualifiers, i think they'll have 128 + reserves.
I think they have a much better chance to fill the European Open which is in Germany in August.
For the UK Open, which starts in nine weeks, I think it's not possible. Only 12 qualifiers have been announced. I don't see taking 10 from each qualifier.
If anyone knows a room that might like to have a qualifier for any of the Opens, here is the contact to get more info: pool@matchroom.com