The only way to create a point with asymmetrical handles while drawing is to hold the alt key when dragging a point, this however creates a corner. Could you add another modifier key (ctrl?) so that it will remain a smooth node but with asymmetrical control points.
Please refer to http://youtu.be/MiS3iqiXuF4 (enable annotations) for clarification on this in Glyphs.
Also http://youtu.be/L-nia5B1Wks is how the functionality works in Illustrator; you can just click and drag on the point again to set the next control point while the point remains a curve.