But what I would like to do is, for example, modulate the parameter within the plug-in and capture that movement in the host.
In the example below I am modulating the Dry/wet' parameter which I have also assigned to Multiparameter 4 (which I have named 'DW' (just to get it into the screenshot).
>>> http://i.imgur.com/AOchNAT.gifYou can see the Easy screen knob move and the envelope slider in Reaper move too, but the envelope points are not written into the automation envelope. I've been advised that this can be achieved if the info passed back to the host has the necessary control / statuses set. (The principle is valid as I have done it with a plug-in from another dev).

