you did not understand my non native speaker english - or it s not enough sleep, or not enough coffee 
trying to define the extrusion direction by the current mouse position seams to be the approach mc neel is targeting, or at least what I understood @pascal is voting for.
so your quote of my post is already a interpretation of what pascal wrote earlier ;-(
i think a pure numeric input independent of the mouse position should be possible. cplane and curve direction can define + / - direction, a (missing) visual indicator could show the +/- direction, similar to _dir command or other UV indicators…
identifying direction by mouse might be an additional option - but should not use the boundingbox-Center as this is counter-intuitive for large curves / complex projects as i show at this post
(with not much resonance)