zap-up-to-char take a prefix argument and a character argument prompted for in the minibuffer. Both functions seems to implement this by using
(interactive "p\ncPrompt string: "). But they behave very differently in
iy-go-to-char only prompts once for the character and uses that same one for every cursor;
zap-up-to-char prompts for a character for each cursor.
I'd like to know why multiple-cursors treats them differently, but above all I'd like to have them both behave as
iy-go-to-char does: single prompt, reuse that character for all cursors.