Although this is not an ideal solution, I recently answered a question that had somewhat similar requirements. Please be especially specific with regard to the version of Flex SDK used, as these things can vary from one to the other.
How to show a line below each line of text in Spark TextArea
I am not aware of any style that specifically indicates the distance of the line from the text (I believe that this may be part of the font definition itself, somehow using the ascension value or something in that direction, m not quite sure how I donβt dug all this style code).
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