The trick is to include the isolated staff in its own StaffGroup.
In your example, it would be something like:
\score {
\notes <
\type Staff = staffa \relative c'' {c4 d e f}
\type StaffGroup = group <
\type Staff = staffb \relative c'' {c4 d e f}
\type Staff = staffc \relative c'' {c4 d e f}
>
\type StaffGroup = singlegroup <
\type Staff = staffd \relative c'' {c4 d e f}
>
>
}
(I don't have Lilypond available here, hope I got it right anyway)
/Mats
>
> This is supposed to have a 2 staff StaffGroup in the middle, and a
> loner Staff above and below the group. However, the StaffGroup seems
> to "adopt" the lower staff, such that you get the isolated staff on
> top (correct) and a StaffGroup of three staves (incorrect - should be
> two staves and a loner).
>
> This is a problem that may come up when one wants to make orchestral
> scores, as the percussion is often not grouped.
>
> John
