Vertical positioning of rests

[Leo Correia de Verdier]

Dear list!

I’m attempting to write a function that would have to move rests vertically 
after line breaking. I would need to move them (grob or stencil) to an 
determined position in relation to the staff, but I cannot find what property 
of the grob determines its vertical placement. Y-offset is zero, both for the 
rest itself and its parent note-column and staff position is not set, even 
though the rests are displayed different positions on the staff. Do you have 
anywhere to point me?

Thanks!
/Leo