Editorial figured bass

[Jean-Charles Malahieude]

Hi all,

I need some help in order to reproduce the enclosed image.

I don't care replacing the parentheses used in the original by brackets, 
but impossible to get only either the opening or closing bracket, even if

\displayMusic {
  \new Staff \with { implicitBassFigures = #'(99) } {
  \figuremode { <[6 99]>4 <[6 5]> <[6 99> <6 5]> } }
}

announces them. It seem like only balanced brackets on the same very 
figure can be outputted, and I don't find my path in constructing an 
"adapted" engraver that would "blank" the missing bracket.

Thanks in advance for any pointer,
Jean-Charles

extract.svg
Description: image/svg