My understanding is that you want to encode two or more events types in the same callback signature. If so, how about a straight forward C encoding as an enum for the type and a matching union for the values? It will allow you to add more event types in the future in a straight forward way. Are there performance concerns?
BTW, you will also need to specify if each callback gets a copy of the original line_dsc (intuitive?) or the left over from previous call (confusing?).