Form of multi-measure-rests

[David Sumbler]

In a piece which is in 21/8 time (7 dotted crotchets) I find that
LilyPond sets whole-bar rests as breve rests.  I should like them to
appear as semibreve rests, as they would if the bars were shorter.

I have tried modifying the stencil using the method shown in NR5.5.3.
 But as it stands this is not successful, because the rest symbol is
now attached to the preceding barline instead of appearing in the
middle of the bar.

How can I get semibreve rest symbols in the correct position, rather
than breve rests?

David