The problem is one (as ITI stated) of bleed... we need it in printing.
I used to do some offset and much larger digital airbrushing - on carpets actually - in my "previous life" before I moved into IT. There is much more complexity to this, but in a nutshell - natural colors always bleed. Our eyes perceive colors with much less specificity than extremes - i.e. black and white.
It's interesting that the substrate itself is less important than the manner in which it interacts with light. If significant light is involved (esp.
projected light) - we will see color-bleeding by design - and our brains smooth everything out. That's why an LCD at ~100dpi looks pretty smooth, while a 100dpi printed page looks horribly pixellated (because it's reflected light only). Also why a 600dpi inkjet print will always look better than a 600dpi laser print (unless it's laminated - which causes bleed to occur despite the pigments being incompletely fused).
If you're using aluminum it should be pretty good, but I would wonder if you couldn't also use similarly perforated mylar sheets which could be printed prior to perforation. Since the solid areas of the aluminum should be highly reflective - you should be able to use the same pore size I would think. Have you tried other substrates in your experiments? I would think at worst this would provide a dithering effect similar to newsprint images. You mentioned using a vinyl I believe - but there are many types of vinyl/plastic substrates - some much better to print on than others.
I'm working on artwork for my GIK panels at the moment. However because of the nature of the fabric I've decided to design only abstract art to print on them since our eyes are much more accepting of blurred images of things we don't associate with actual visual memories. With pictures of people or objects we see in daily life - our brains seem to spend much more time figuring out "what's wrong with this picture".
Since I'm using my livingroom as my HT - I can't see Star Wars posters going up there without announcing my 'geekdom' to everyone that walks in the front door (no matter how accurate such a branding might be).

But I would be very interested in seeing some pictures of the final results of your printed aluminum panels posted here.