This works fine for cat or curl commands which have a defined end point, for ffmpeg decoding the rtsp stream no return code happens until the stream is stopped, but I do get one nice complete image that appears to be a concatenation of all the images in a single message -- the output buffer from about half a minute of running was like 5MB.
I think this approach is doomed, It appears ffmpeg outputs complete jpegs for each frame ~5/sec for my stream but the exec node breaks it up into multiple pieces if the size exceeds 64K.
Now that its dark out, most of my images are ~62K so it mostly works, which has interfered and with and confused me during testing. 
