I could very well be wrong, but I believe when Sir Thomas, in an earlier chapter, spoke of giving her the provision of a gentlewoman, he meant some sort of dowry - probably less than what he would give Maria or Julia, but something, at least.

Of course, in that same chapter (when they were discussing bringing Fanny to Mansfield,) Mrs. Norris opines that Fanny, if she were brought up at Mansfield and introduced to society as Sir Thomas Bertram's niece, would be able to make a decent match - yet, as you point out, until very recently nothing was done to introduce her into society.