Tweaking endpoints of spanners?

[Joe Neeman]

Is is possible to tweak spanners by adjusting the endpoints 
individually? For example, if I have a glissando from
bes to b, it gets obscured by the staff line. I'm looking for something like

\once \override Glissando #'endpoint1-extra-offset = #'(0 . 1)

The same question applies to hairpins; I have a few situations where I 
want to shift one end of a hairpin up or down without affecting the 
other end.

Thanks,
Joe