Dan McMullen has recently posted a tutorial on the Reaper forums, detailing a few techniques on how to map a standard MIDI control to a limited range of the much higher resolution Pitch bend control, in order to get precise control over the ValhallaRoom DECAY slider (and other sliders). You can see the post here.
Using High Resolution Automation in Reaper w/ValhallaRoom
Written by Sean Costello

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woah, now that is useful! Thanks