I thought so too, and I was looking at Custom for a long while-- but finally realized I can't have ascension or direction independently influence the brush settings (radius, jitter, etc). The Custom input says it's a "mixture of pressure/speed/whatever". I think I need more than a "mixture", but an actual relationship between ascension and direction, the AOA or maybe Angle of Incidence, to know whether the tool is moving parallel to itself versus side-to-side, etc. If you have a tilt-enabled stylus, play around with the Classic/Marker Fat to see what I'm talking about. You might want disable the declination on elliptical dab ratio so that it stays at 10:1.
Basically, with the Classic/Marker Fat-- the dab angle starts off at about 90 degrees (actually 113 but 90 works better IMO). With the Ascension affecting dab angle, the dab angle will always be parallel to the ascension. With the stylus at 12 o clock, ascension is 0, so dab angle stays at 90. At 3 o clock the ascension is +90 so the dab angle becomes 180, etc. So, you'll always get a thin line when "dragging" and "pushing" regardless of the ascension.
So right now the only way to get this effect is to work with dab angle and a large dab ratio.