MIDI Designer is 100% bidirectional (meaning it responds to MIDI input)... that said, I don't know how to get Reaper to send values for the faders.
Compared to the Reaper forums, we're tiny: you might want to try asking there. If you get some answers that are worth sharing, please come back and share. Also, if you have any comments or questions related to this thread, please comment on this answer.
Thanks!