I ended up having two of them hold rockets because of the dearth of good looking crew arms in the Dwarf Cannon box (seriously). I also forewent resculpting any of the beards. I decided it was a better expenditure of effort to give them brutal looking facial coverings. To that end, I did uncover a few neat techniques for doing this.
First (and most generally applicable) is that an easy way to make rivets is to dice up little tiny pieces of semi dried greenstuff and glue them in place. I say semi dried because (while unintentional) I found that I could still shape them a little after they were glued in place. I used this to massage the rivets into slightly rounder shapes. This resulted in pretty tiny rivets though, which was exactly what I wanted. I'll post again once I've painted them so you can see how the technique turned out.
Second, I wanted to mimic the scale mail facial armour from one of the Hellcannon crew. It's a little hard to see in this picture, but I actually chopped up plasticard into small rectangles and glued them down to create scale armour hanging out to cover the bottom half of this guy's face. It's hard to see because I used clear plasticard since it was the thinnest I had.
I don't have anything interesting to share about the third crewman. I just free-sculpted a skull mask onto him. It's not the greatest thing in the world, but I think it'll do once it's painted.