Or a defcustom, since it seems that the usual approach is to override the behavior unconditionally.
A defcustom seems too risky for this. A user can unknowingly copy that var from somewhere and then end up killing modified buffers without confirmation.
Add a new defun that just sets the new optional arg to non-nil value is less risky as the user would then be knowingly calling the risky variant. The default kill-matching-buffers will still remain safe.
What do you think?