I haven't tested this but it may work. Try nesting two inline styles and use background colours and padding/margins to define the border.
As a matter of fact the XSL:FO specification allows border attributes for the inline element which apparently is not supported by FOP. I thought they're might be a work around to draw a border around a word placed in a sentence. Maybe someone successfully overcame that obstacle... If so, i would appreciate some help very much...
ie. the first style with a black background, the second (inner) style with a background that matches your page background and margins to hold it in the required border width.
Here's an example. The margin syntax may be wrong, it's early morning and I can't remember what it's meant to look like. I'd be interested if this works.
<fo:inline background-color="#000000"> <fo:inline background-color="#ffffff" margin-width="0.5cm"> Text in here </fo:inline> </fo:inline>
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