My set up only allows for a passive system. At least, without a LOT of mods... I was worried about GFO solidifying, so I kinda scrunch it up ever so often. But, the point about reduced flow has probably talked me out of it. Worth asking, though, eh?
The only reason why you need to tumble GFO is to make sure that it does not clump. You can accomplish that by mixing 1 part GFO to 2 parts GAC. The extra GAC will prevent the GFO from clumping (e.g. ChemiPure Elite). That's conservative and you can also do 1 part GFO to 1 part GAC if you like to live life on the edge. The major drawback is that GFO is exhausted much more quickly than GAC so you will spend more money (on GAC) for the convenience of a one pot solution.