The direct approach with Amex works well. Simply call them up and ask for a CLI to $100k+ on one credit card "or the maximum limit I qualify for." They'll request 2+ years of tax returns via 4506-T and will grant it if you qualify.

It helps to have at least one existing card at that level with another bank, but some lender has to be the first, so it may as well be Amex.

Then it's on to a $200k+ limit on one card, which at least one member on here and/or FW has, as of a few years ago.