I see three possible solutions:
1.) \override Voice . MultiMeasureRest #'stencil = ##f (or \override Voice . MultiMeasureRest #'transparent = ##f). (This can be switched on and off. You can even put it into the default context definition, so you only have to write it once, or you define a command for it.)
2.) \with { \remove Multi_measure_rest_engraver } for the corresponding (or all) Voice contexts. (Which is a global solution, at least per voice - but it should be more efficient, if it matters...)
Since, AFAIK, there is no collision checking between MultiMeasureRests and elements in other voices - and, thus, using the #'transparent = ##f should not break anything, compared to removing the engraver - I'd probably almost always prefer either option 1 or option 3:
3.) Use "s" instead of "R".
Cheers,
Alexander