If you get a chance, try implementing a Scheme engraver - this type of override can wreak panic with spacing changes in LilyPond, whereas a custom engraver will survive changes in spacing.
Did you ever implement this as a custom engraver as Mike suggested? If not, I'm attaching my attempt at a solution.
So that a score could contain both slides and ordinary note-to-note glissandi, I initially tried to create a custom FingeringSlide grob, with limited success. Here, I work with Glissando, adding a property to switch the slides on and off:
\override Glissando #'to-fingerings = ##t
A glissando will still be drawn between every note, so if there aren't enough fingerings to align the glissandi to, the remaining ones will appear near their note heads. The solution to this limitation is to set glissandoMap, as I do in the attached examples. (This is also useful if there is a slide along with an ordinary change of fingers, since the engraver as it stands will draw lines between all pairs of fingerings.)
Sorry about replying to an ancient thread like this. I hope you find this useful!
-David
fingering-slide-engraver.ly
fingering-slide-engraver.png