Staff grouping

[Jérôme Plût]

Are there any other ways to tune the vertical spacing between staffs
other than the \paper block? What I want is the following: on the same
page,
 - first a score A (single-staff, no groups) with successive staffs
   extremely close together (about 2 or 3 staff lines of separation
   should be enough),
 - then another score B with successive staffs normally separated. (B
   is a StaffGroup score but that should not change anything to the
   problem).
Ideally the last A staff should also be very close to the first B
line. (In my application, B is a real score and A are blank lines for
comments etc.)

I tried putting two \paper blocks, one before A and one before B, but
it looks like the first one is then ignored. Is there any other way to
do this?