[Metalstorm]

Can you stuff things into those cowls? I've never ridden without a quart of oil to get me home in case of a leak and a can of fix-a-flat and there's not much room under the seat for that stuff.

I don't know if you can "stuff" things into the cowl but according to it's description on Honda's website, it "Increases lockable cargo capacity to 4 Liters."

 

[Metalstorm]

I noticed it also says something about possible extra dealer fees for "installation." Just how tough is it to "install" one of these?

That has to be a generic disclaimer.
There shouldn't be any "installation". It replaces the rear seat. Turn key, remove seat, put cowl in place of seat and snap it in just as you would with the seat.