Who would be the star writers whom we could invite, who would attract legions of Catholic writers out of their solitude and onto airplanes, and then to a faraway city for a convention? Who are the Catholic writers today whose work will be read in fifty or a hundred years?
I think the only way to answer the second question is to look at it as Josiah suggests in a comment. First, who are the writers today whose work will be read in fifty or a hundred years? And next, which of them write Catholic books?
Of course, that's still a much different question than, "Who would be the star writers whom we could invite, who would attract legions of Catholic writers out of their solitude and onto airplanes, and then to a faraway city for a convention?" Dame Mary Higgins Clark, Order of St. Gregory the Great, might attract legions of writers, sub-legions of whom might be Catholic, but that may not be the sort of writers' conference intended.
To really attract legions of Catholic writers, I think Tom Monaghan should host a "Survivor"-style weekend, where the winner gets a million dollar advance (hence Monaghan) on his first (or next) book. Also, the hotel bar should be guaranteed to have a minimum of three open-minded agents at all times. (Make it an open bar, and registration will be full within a week.)
In my very limited experience among Catholic writers, I've noticed a (by no means universal) "Catholic first, writer second and only for the glory of God" tendency, which itself may have something to do with the dearth of outstanding Catholic writing, but which also works against legions of writers going anywhere. Someone who writes only for the glory of God is likely to find that he's not doing much writing for money, and someone who doesn't make any money writing is not likely to spend several hundred dollars to network and learn a little about writing.
Then there's the factional problem. To the extent ritual purity is more important than the craft of writing, the conference can't invite someone like Fr. Andrew Greeley or Scott Hahn without losing a chunk of your intended attendees.
I suppose the answer to how to set up a conference lies in teleology: What is the intended end for which attending the conference would be the means?