Making an object not take up vertical space?

[Marcus Macauley]

Is it possible to make an object (e.g., a text markup) not take up any  
vertical space?

I've been using Mats' tip for inserting extra vertical space, to good  
effect, e.g. (customized):

pushup = #(define-music-function (parser location padding) (number?)
        #{
                s1*0^\markup { \pad-markup #$(* 0.5 padding) \null }
        #})

pushdown = #(define-music-function (parser location padding) (number?)
        #{
                s1*0_\markup { \pad-markup #$(* 0.5 padding) \null }
        #})

and then
\pushup #3
wherever I want to insert 3 staff spaces of vertical space above a staff,  
or
\pushdown #4
to insert 4 staff spaces of vertical space below a staff.

But there's one place in the score I'm working on where Lilypond makes  
unnecessary vertical space for a text markup (I reported this to  
bug-lilypond), and I wish I could do the opposite, like:

\pulldown #3
or
\pushup #-3

to insert -3 staff spaces of vertical space above a staff, thus "pulling  
down" the staff above it.

I tried changing 0.5 in the above definition to -0.5, but it didn't work,  
even with very large numbers.

So the thing to do would be to prevent the offending text markup from  
being accounted for in Lilypond's vertical spacing routines.

Is this possible?

Marcus