Proportional-Notation Durations

[PMA]

Hi List.

I'm looking for a LilyPond way to specify note duration
in a Proportional Notation context using exactly _one_
stemless notehead type.

Perhaps the first option is a mid-level horizontal line
extending distance-X from the notehead.  (The space
following would indicate silence.)

But I want ask: has anyone done this instead with the
_hairpin_ -- attached to a note-head via pitch params,
tapered to a param-specified length, and filled?

If not, or in any case, any other ideas?

Thanks in advance for your time.
Pete