Couple of things...it kind of sounds like the media (rocks and bio balls) might be submerged? Can't tell from the description, but submerged wouldn't be good.

A shower head would clog pretty fast I'd assume.

It sounds like you're over thinking this.

A trickle tower can just be a pile of rocks with water flowing over it. You can get some heavy steel mesh, form a cylinder, fill with rocks, run water to the top.

A shower filter can just be a few stacked planters with water flowing over it. The spray bar at the top is just PVC with some drilled holes.

I assume you've seen these, what don't you like about them?

I'd like to provide some links but this site doesn't allow me too and I also can't mention the sites apparently.

I agree that a shower head would not be a good idea.
As a diffuser I might think about using a plastic tray with holes drilled on the bottom to allow the water out as fast as the pump is moving it to it. Allow this to fall on to a sheet of matala and than fall onto the other media. The other thing that will improve the performance a lot is to put a air stone or stones in the bottom. The drainage pipe at the bottom of the devise has to be large enough to drain the water back to the pond faster than you are pumping up to it.
Mike

Aha - keep water flowing out the bottom - thats the missing bit of info I didn't have - thnx.

Did see the stacked planters, but where to put it or how to construct something suspended above the pond was problematic. At this point Im thinking quick and easy - without having to reconfigure my pond. Due to way it was built (kind of ruin-like with 2 foot walls around 1/2 of it - patterned after celtic holy wells) there is n't much of a waterfall area so having the filter structure next to, or behind the wall is what I was thinking - which means it would need to be contained. Was thinking the water could be returned to the pond via some old looking antique spigot or metal tubing from the tank, through the rocks, and back into the pond.

Although as Im thinking about it, I could build up something more vertical (in what waterfall space there is) with the rocks with water going through it - more like a waterfall or springlike effect with water bubbling through the rocks.

But you see... one other factor is that all my rocks around the pond are limestone, and after 5 years of collecting rocks and building this thing, someone recently told me that limestone is BAD. PH here in Wisconsin is almost off the charts anyway and apparently limestone washes off the rocks and into the pond and makes it cloudy - not good. It does seem to be cloudier after it rains.

So, if I wanted an actual waterfall I'd have to get other rocks, at least in the waterfall area, to keep the running water separate from the limestone. Just seemed to be too much of a PITA.

Although Im not sure how much creedence to give this - my previous small pond had limestone around it too, with water running over it, and it wasn't a problem. Once they get all mossy and weathered maybe it isn't a problem?

Limestone can be problem for some ponds depending on the source of water for them. Based on what you say about your PH I would guess that your water source is from well water. Well water in this area is almost always from limestone wells. So having limestone in your pond is no big deal. The biggest problem with limestone is that it is soft and can wear fairly fast. The other problem is that with our freezing weather it will sometimes tend to break apart from freezing.
Mike

Without seeing your pond I can't really offer any good suggestions. So I'll stick to some general stuff...

The trickling spring deal running over a vertical wall is basically a trickle tower. And is excellent for most water gardens. Only high fish load type deals really need the high ammonia conversion rates of a shower or large trickle tower.

You could test the limestone by placing it in a bucket of water and see what happens. You can even scrub it a bit. See if it clouds the water. Generally I don't have much faith "somebody" telling me something is actually going to be true. Smart money is always on they don't know what they're talking about.