Is there a way to determine the height of the current line? More specifically, after the hbox comprising the current line is finalized, I'd like to determine it's height (and perhaps also depth) and write this information to an auxiliary file. The application I have in mind is placing something in the margin which is vertically aligned with the center of the current line, rather than with the baseline of the current line. I'm primarily interested in the case where the current line is a displayed equation.
(This is a more focused version of this older question.)