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This already works. LoginForm::displaySuccessfulLogin() always has the logged on user name as a parameter and uses $wgOut->addWikiMsg which processes magic words.
Only("You are now logged in to translatewiki.net as "$1".") is fed into displaySuccessfulLogin() in core and svn extensions.
I only checked the link that Raymond gave me here. Is there some list for other pages besides those that support GENDER.