The battery effect is quite real. You ought to look into what's been posted here, historically.
Your system isn't immune to it either: as long as a current can flow through the coolant, between two different metals, you have it going on.
The impact is that it accelerates normal corrosion (aka rusting).
As you know, Aluminium will have a larger impact on the galvanic effect, because it's so dissimilar to copper/brass, on the galvanic chart.
In general, you want to avoid using aluminium in combination with copper/brass (the copper/brass combo is OK). If you must use them, then use an anti-corrosion agent, and flush it out every six months (at least that's the recomendation for a closed loop system!). In your case, it'll be trickier, because you're running an open loop, and you have a (relatively) significant loss of coolant.
If I was you, I'd make my own shower head. Get some copper fittings, copper sheets (
www.onlinemetals.com ) and solder away. Then punch some holes in the copper sheet for the spray. You'll then have your standard NPT connection, and worked around the battery effect. If possible, make the plate removeable, for easy cleaning (a gasket/o-ring, and 6 screws ought to do it).
I liked your filter story. I'm still trying to find a small and cheap, low pressure filter, for my Cascade SS block.