centering objects on (over) barline

[Kieren Richard MacMillan]

[ Mac OS X 10.2.8;  Lilypond 2.0.1 ]

Howdy!

Is there any way of centering something on/over a barline explicitly 
(i.e., as opposed to putting it over a note and then using extra-offset 
to guess at the distance, which might change depending on layout)? 
There are lots of situations I can see needing such a thing (e.g., coda 
and D.S. signs, fermatas, breath marks, etc.), and it would be great 
not to have to worry about alignment.

Thanks,
Kieren.