Not really possible. In order for the login password to be passed to the game it has to be in cleartext in a file included in the UI folder - the UI simply doesn't have the ability to pull the data in from anything else.
I honestly recommend against people using login windows in EQ2 for that reason - having passwords in cleartext like that is always a Bad Idea. Fetish has a similar login feature dating back to Zonx that if not for a few requests I had planned to remove.
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