If you want to ratsnest a single net the only option is currently to type
the command followed by the netname in parentheses.
If you need to incrementally make adjustments to a polygon's shape, there is
no way to simply "repeat" the last operation. All that typing can get
tedious. And slow.
So, give us an additional drop-down menu option such as: "Tools > Ratsnest
(.) " where Eagle waits for the user to mouse-click on a single net to
operate on. Or, give us a syntax for typing the "ratsnest" command, to be
followed by a mouse-click on a single net to operate on.