Mensural time signature + proportion

[Francesco Spiga]

Hi,
I'm typesetting Renaissance music and I'm trying to show a mensural time 
signature followed by a proportion, i.e. "cut 'tempus perfectum' 3/2", as here 
appears on page 2:

http://javanese.imslp.info/files/imglnks/usimg/1/11/IMSLP78902-PMLP43368-
Vittoria__Missa_Quarti_Toni__MD_Proske__Cantus_mono.pdf

A first solution I have found is this:

\once \override Staff.TimeSignature.style = #'neomensural
\time 3/4
s2.
\time 3/2

Unfortunately it gives a too wide space among time signatures.
Has someone any suggestion?
Thank you in advance!
F.