Do headset and frame metals need to match, or will a steel/aluminum headset get stuck on an aluminum/steel frame?
Seatposts have a relatively long length inside the tube, and no way to get a tool to the bottom of the post to hammer it. If you have a proper headset remover tool, I doubt a headset would ever get so stuck. (Even without the proper tool, it's unlikely to be a problem.)

Also, I don't believe there's such a thing as a headset with an alloy bearing race. So if it has alloy cups, they probably have replaceable bearings that fit inside it, so the cups shouldn't ever need replacing.