I thought I could help, but apparently my scheme-fu isn't good enough
for this. I first thought I could beat \Balloon_engraver into doing what
you need, but there's scant information on it, and I don't know how to
change the size of the box it creates.
Then I thought I could just use markup and extra-offset to move the box
where it needed to be, but again, I lack the understanding of how
extra-offset works to be able to use it.
Perhaps, however, with those two suggestion you, or someone else may be
able to get the results you need.