You know, this might or might not be how you want to go about the painting Phruby, but when I was referring to a metal look, I really didn't mean for it to be overly metallic, just the texture to not look like concrete.
What I would suggest is the walls having some sort of "border" on them - you could use strips and glue them to the top and bottom of it - almost like what is done with base boards on a real wall in a house.
For example, on the junkyard wall if you want to look at that you can see the "border" that I made on it -
When painting something like this - I've found that a base coat of a Krylon gray primer works best... then you can use another Krylon gray paint after it dries, and then add a bit of weathering to it. For weathering, all you need is some water, acrylic paint and a couple brushes. It's very easy if you have the right kind of base paint coat down first (again I'd suggest Krylon but the Rustoleum might work too).
Also - I've found that using the cheap 99 cent paint from Walmart is a huge mistake, it's better to buy the Krylon stuff that costs about 3 or 4 dollars per can.
Just ideas though, how you go about this will be up to you - I wish I could have done more on the wall, but as you know my situation I was afraid to possibly over commit on it.