defining a \markup-only context ("Staff"?)

[Kieren MacMillan]

[Lilypond 2.11.49]

Hello all,

I'm trying to define a context which will act like a Staff, but will  
only contain \markup objects (attached to skips). I may need to put  
it above the rest of my GrandStaff, or possibly embed it (e.g.,  
between the strings and the piano).

I've started with

        \context
        {
                \type "Engraver_group"
                \name "ScriptLine"
                \consists "Output_property_engraver"
                \consists "Axis_group_engraver"
                \consists "Text_engraver"
        }

But regardless of where I put my ScriptLine context, it doesn't  
trigger vertical spacing correctly -- in particular, it doesn't seem  
to have any vertical extent, there are lots of collisions (i.e., it  
overlaps with adjacent Staff contexts), and the \markup text (or  
entire context?) always floats to the top of the GrandStaff that  
contains it.

Without posting an example (which might be hard to reduce), is there  
anything someone can see or suggest?

Thanks,
Kieren.