Would a sheet of aluminum help with mounting a <20lb computer monitor to wall without studs?
Aluminum wont help much, unless it is quite thick.  It will bend easily, without distributing the weight evenly over the extra anchors.  

If you go with a non articulating mount, so the monitor is against the wall, you may be able to get away with strong drywall anchors.  With an articulating mount I wouldnt risk it.

I would get a small piece of cheap but decent looking wood to span the studs, and screw it on over the drywall with 2 screws in each stud.  Then paint it to match the wall.  Screw the mount into that.

A 27" monitor may not weigh all that much, but the movement and lever force from the arm mean whatever is holding it needs to support a lot more than 12lbs.  Personally, I wouldnt want to have it fall on my foot, and then have to buy a new monitor!