Brian, morning.... This has been an idea floating around in the gray matter area for awhile.... Hasn't been put into practice that I know of...
The drawback appears to be the RF drain has to be open all the time.... Water, for steam, will have to be put in a baking pan of sorts... I don't think that would be a major deal.... Seems this "steaming meat in a smoker" idea has exploded into some ritual.... I don't do it...
A well designed RF smoker will control the temp perfectly... Temp swings from opening the door are normal and expected... Thermal mass will help with reducing swings in temp... The RF plate does not need to be perfectly air tight... The area at the end of the plate will be the preferred air flow path.... I think a removable plate would be the cat's ###... The RF plate "V shape" could be done at the shop in a press brake... 2-3 degree bend "V" would be enough to get the grease moving to the center toward the drain..
Click on pic to enlarge