On the REAPER forum a new user was asking how you could split the audio into various bands and process each independently. Here is a project I’ve set up to do just that.
The source audio track is sent (master send disabled) to 6 other tracks, each with a ReaFIR plugin to define the bands. Subs, Lows, Low Mids, High Mids, Highs and Ultra Highs. After that is a simple compressor for each band. You can insert any other plugins you want for each band: Saturation, stereo width etc. You may find this to be a good starting point for mastering.
Here’s the Project File: http://dl.dropbox.com/u/1053122/multiband%20template.RPP
Let me know if you find this useful or have any problems with it.