Since it explicitly requires a rhythm, the performer would be likely to attempt to be precise in its execution. The desired outcome is to have an unmeasured flurry of hits all the way through the figure. My eventual solution was as follows:
The 32nd notes with extended beams suggest that the figure is unmeasured and very fast. The whole note defines the starting pitch and duration of the gliss. The text clearly states the requirement. Result: no confusion.
Update: John pointed out that having the mixed roman and italic text looks odd, especially following the line. I think that a better solution is to treat the text as a technique and have it all in horizontal roman type as follows: