Not ideal, but missiles can be assigned to auto groups.
If you have a shockly (partially or fully built) and you press alt+1, any shockly fully built can now be selected using 1.
If you auto group shockly to 1 and eos to 2, you can do 1,F,2,F and that'll do what you want.
I tweaked my settings so I am not sure about he default, but I think auto groups carry over game to game, which can be annoying.
The other potential problem is that auto groups will add any fully built shockly from any silo, so if you have two silos with a shockly each, 1 would select both and you could "overkill" if both are in range of the target.