#(add-simple-time-signature-style 'topsy-turvy
   (lambda (fraction)
     (make-rotate-markup 180 (make-compound-meter-markup fraction))))

<<
  \new Staff {
    \time 3/4 f'2.
    \override Score.TimeSignature.style = #'topsy-turvy
    \time 3/4 R2. \bar "|."
  }
  \new Staff {
    R2. e''
  }
>>