How can I shorten the space for a final single note

[Robert Blackstone]

Dear all,

I have typeset a small example from a theoretical work of Lorenzo  Penna.
It has 2 sections, each consisting of three bars separated by a double barline. 
The time is 4/2 and  bars 1 and 2 contain mostly half notes. In each section 
the final note is a \breve. The problem is that Lilypond gives these single 
final notes the same horizontal space as the four halfnotes in the first two 
bars. 

My client does not like it and wants these final bars shortened to at most half 
their present length with the \breve centered.
Can anybody tell me  how I can realise this?

I tried \set Timing.measureLength = #(ly:make-moment 1/2) and that works but it 
spoils the result by inserting three empty bars which I do not know how to get 
rid of. (The law of preservation of misery in full action.)

Thanks in advance for any advice.

Best regards,
Robert Blackstone