Moving glissando ends

[David Sumbler]

For harp ad lib. glissandi, I am trying to produce a continuous line
which zig-zags across 2 staves.  I thought I could do this by placing
notes on alternate staves at the end of each "zig" or "zag", and then
putting glissandi between them.  Then (I thought) I could make the
notes invisible, and change parameters of the glissando lines so that
they actually go to the centre of each (invisible) notehead.

Making the notes invisible is easy enough, but try as I might, I can't
seem to make any change in the printed length of the glissandi.  I have
tried altering the 'gap' parameter and 'padding', as well as a few
other less promising items.  But nothing seems to make any difference
to the printed length of the lines, which therefore have gaps between
them.

I want each line to go to the centre of the invisible notehead, so that
I get a continuous line.  Of course, the first and last lines only want
to be full-length at one end, because there is an actual notehead at
the other.

How can I achieve this?

David