Well as you discovered the mono button on the master is a monitoring only function. It will not affect your mix.
Place a plugin with Mono option on the the master bus insert.
I'd suggest a suitable Reaper stock plug..sure there is one but as I closed my DAW just now I couldn't tell you which one!
A1 StereoControl is a third party free vst that can certainly do it.
http://www.alexhilton.net/A1AUDIO/in...1stereocontrol
EDIT: In fact you can also do it without any plug, switch the channel pan to dual panner (right click I think?)....put both to centre.