demo.rb

require './extension'

class A
  def foo
    puts "Success!"
  end
end

class B
end

B.superclass = A

B.new.foo # prints "Success!"

extension.c

#include "ruby.h"

VALUE superclass_set(VALUE self, VALUE super)
{
    RCLASS_SUPER(self) = super;
    RCLASS_M_TBL(self) = st_init_numtable();
    OBJ_INFECT(self, super);
    return super;
}

void Init_ruby_super()
{
    rb_define_method(rb_cClass, "superclass=", superclass_set, 1);
}