OK, so I have a vague answer to this question - only because I've been searching for a very long time (since reason 4) and FINALLY found a solution.
First of all - as of February, 2015, Propellerhead Reason Support (via facebook) says that this CANNOT be done directly in Reason according to their response here: Facebook thread link I found on Google

Now, that being said, I DID find a workaround. I recently found a free (for personal use) program called MIDI-OX, which lets you look in the MIDI-OX monitor at the value of the velocity registered when you play the keyboard (from 0-127 range), and then "Map" new Velocity-Waves as MIDI-OX output, then you set your Reason input to MIDI-OX.
In order to set a DEFAULT VELCOITY for ALL MIDI input, you would set up a "NoteOn" filter, which checks for a min and\or max velocity (from 0-127) and then remaps midi OUTPUT velocity from the virtual MIDI-OX device, to a specific OUTPUT velocity. See the example screenshot below

You would then set your MIDI-INPUT device in reason to "MIDI-OX"
You can find more information here:
MIDI-OX.com
Hope this helps!