Good catch Jeffos.., I had also tested to see if it was a REAPER bug, but I was testing sending messages to "/track/1/fx/1/fxparam/5/value" which worked just fine. I did'nt think of sending messages to REAPER actions.., this info should help the devs find the cause at little easier. I'll go ahead and write up the BR.
Update:
Ok I wrote up a BR and posted it in the Issue Tracker here
http://forum.cockos.com/project.php?issueid=4589 ..., maybe when you have a moment you can set the status to confirmed.
Thanks!