Woz,
If you've built a system in your blue barrels that adequately does the job of mechanical and biofiltration then, strictly, you don't need any more filtration after the skimmer and you could simply use it for what skimmers were really intended for - skimming the surface of any floating debris, such as leaves, and collecting them in the basket, anything more than that would be a bonus.
That said, you can use the flow to do anything you think might be an improvement. As you say, you could add an extra form of biofiltration; you can never have too much of that. You mentioned a Bakki Shower, these are expensive to buy but, since you've built your "blue barrel" system, there are simple DIY showers that you could make.
Removing fines and improving the water clarity is another possibility. Filter wool is good for that so my choice would be to build a shower with some filter wool laid on top of the media in the top tray. If you do that, I would suggest that you don't entirely cover the media so that, if it blocks, the water can flow over the edges and then through the media rather than over the sides of the top tray.
There are plenty of designs on the Internet using planters as trays, here are a couple that I've just found. I don't like the "bathroom towel rail" using pipework method of holding the planters apart in the first one. The second is a bit plain but shows how the planters could be nested together with the spacers inside.
