You won't believe it, but WordBuilder could have been working in Reaper for years. All thanks to the marvelous plugins by Xlutop.
Download the demo of the "Chainer" VST plugin here (you should better buy it, because otherwise you can't save settings in your Reaper project, but be sure that everything works in the Demo first):
http://www.xlutop.com/html/downloads.html
Download the free MFX plugin wrapper here:
http://xlutop.com/buzz/zip/dxshell_v1.0.4b.zip
Here is a step-by-step guide of all necessary steps:
-) install Chainer
-) from dxshell_v1.0.4b.zip extract "mfxshell.dll" and "shell2vst.exe" (this program is located in shell2vst.zip)
-) place both files in your installation directory of WordBuilder (i.e., the folder which contains WordBuilder.exe and WordBuilderMFX.dll)
-) drag mfxshell.dll onto shell2vst.exe and a terminal window should appear => IMPORTANT: drag-and-drop onto exe must happen in target folder (i.e., East West Word Builder directory) otherwise Chainer won't find the plugin
-) this creates a subdirectory called "MFX" and in it you will find "WordBuilder [MFX].dll"
-) cut "WordBuilder [MFX].dll" and paste it into the "WordBuilder" directory (i.e., one directory up)
-) start Reaper and create a new track
-) add the following FX in this order: Chainer VSTi (Xlutop) -> Kompakt/Kontakt
-) in the Chainer VSTi click the arrow in the lower-right corner and select "Options / Plugin directories..."
-) delete all directories from the list and add the path to WordBuilder, e.g. C:\Program Files (x86)\East West\WordBuilder
-) Chainer VSTi consists of 10 rows, you should now be able to click the arrow to the left of the first row and select "Instruments / WordBuilder [MFX]"
-) click the button titled "1" below the label "Editor" and the GUI of WordBuilder opens
-) load a Choir Multi in Kompakt/Kontakt
I tested this method with different operating systems and versions of Reaper and it worked every time. It seems to run stable and Reaper projects are correctly restored. What this basically means is that if you manage to set WordBuilder up in Chainer, it should work in all future versions of Reaper too!
@EvilDragon: I think you can finally start composing epic choir passages again without the hassle of virtual MIDI cables.