Zemo should be used as a small (non speaking) cameo.
Imagine if these were Nazi bad guys, and Zemo was in the scene some place
Like here:
or here:
or here:
or here:
or here:
Imagine a scene like this:
Hitler is talking to the Red Skull, in the background some troops are doing stuff. One of the Nazi soldiers in the back could be Zemo.
He wouldn't have a speaking part, so that any actor could be cast and wear the mask. Later, when Zemo has a bigger part and a speaking role the real actor could be cast.
This would give Zemo a small cameo that wouldn't take away from Red Skull or Captain America, but still show that he was there.
He could be shown more than once too, still only as a small camoe and no talking.
For example, we see Zemo in the background behind the Red Skull and Hitler doing something. Later when the everyone is fighting we could see him in the background doing something (perhaps running away to the Amazon).
The point is, because Zemo wears that mask he could be shown without being a main bad guy and without having to actually cast him yet. Because of his mask he would be visible.
Other "human looking" bad guys can be used maybe. Like a young Strucker or Zola tortures Captain America in one scene. He wouldn't be a main bad guy, just a quick cameo. But still I say just stick with a small non speaking Zemo cameo.