seconded, this has pissed me off quite a few times when i wanted it but didn't have it, never got around to posting it as a "bug"
It would be nice, but it's not really a bug (and yeah I got that you had it in quotes ). Jump sustain works directly from input, so when you press Jump control, it can calculate how long you're holding down the control. When you use Jump action, there is no control involved, and so there is no way for it to know how long you have the control held down, or if you even have it held down at all.
Of course David might be able to work it into the behavior if you were to ask. As far as I know though he's still busy with school at the moment.