I just a released a fairly simple but powerful little plugin that allows you to redirect and modify MIDI messages by applying custom response curves. The plugin can be downloaded from the product page. The product page also includes some more detailed instructions on how to use the plugin.
To use the plugin, you just place it somewhere in your FX chain and select a MIDI input source from the top-left dropdown, as well as a MIDI output destination from the top-right dropdown. Then you can modify the curve graphically and observe the effects. During playback, the GUI will show incoming values and how they are being transformed, as shown in the second screenshot below. The plugin can be chained as well in order to affect multiple types of MIDI messages in a single FX chain.
Hopefully some of you will find this plugin useful. It's a free product, and I can't make any guarantees on maintenance or fixes, but you're more than welcome to reply to this thread or contact me directly if you run into trouble or have any feedback.
The source can also be found on github. If you're familiar with the platform, feel free to raise an issue there as well if you encounter any bugs.
Screenshots
General usage screenshot, showing a curve that will use velocity values to modify pitch bend:
Highlighting how the GUI displays the last received MIDI input in real-time:
