I was sufficiently curious about the “bounding rectangle” (and a literal interpretation of the pictures in the engraving options) to do a quick check by exporting a page with some lyrics as an SVG file, so it is clear where the text is on the page.
The “bounding rectangle” seems to contain a standard allowance for ascenders and descenders whether or not they appear in the line of lyrics - which is a good thing, because otherwise the line spacing might be visibly different for multiple lines of lyrics depending on the happenstance of the letters and punctuation in the text on each line.
That seems to imply it would be simpler just to specify the separation between the baselines for the lyric lines (as in Finale) and forget about the bounding boxes. For example if somebody wanted to set many lines of lyrics as tightly as normal block text, it would then be obvious what the spacing should be, without bothering about the size of the bounding box which (presumably) depends on the X or x height (not the point size) of the particular font, or whatever.