Glissando fall/rises with no termination

[Benedict Singer]

Hi again, another Glissando related question,

This issue was brought up a while ago with the feature request dealing 
with falls, but I'm revisiting it for a piece I'm working on; the 
section in question is for pizz. cello, and I need a way to indicate the 
pitch rising or falling after plucking the note. If there's a better way 
than using \glissando then I'd love to hear about it, but until then 
this is what I have:

glissFall =
   #(define-music-function (parser location note) (ly:music?)
       #{ $note \once \override NoteHead #'transparent = ##t \once 
\override Stem #'transparent = ##t \relative c { f,8 } #})

varVI = \relative c' {
   \clef bass
   \time 3/4
   \override Glissando #'style = #'zigzag

   \glissFall f,4*1/2\glissando r2
   \glissFall c'4*1/2\glissando r2
   \bar "||"
}

I'm looking for tips from the Scheme/LilyPond pros to make this a bit 
better. In particular, the ending note is currently hard-coded, so the 
gliss down from the F is much shorter than the one down from the C. Is 
there a way to use the note that \glissFall takes as input and modify it 
to construct the invisible note somehow? So that whatever note you 
\glissFall from, the glissando goes to an invisible note an octave, or 
two octaves, or some fixed interval, below the original note? The other 
thing is that this requires the input to have the duration scaled 
appropriately already so that the invisible eighth note can fit in - can 
this be done in the function too? The second issue is less of a concern 
than the first, since I anticipate using \glissFall on a wide variety of 
starting pitches, and having the falls be uniform in length would be 
excellent.

Thanks so much!

Benedict