Attached is a patch to add this feature directly into YARV without a
hack.

Usage:

[pbrannan@zaphod md]$ cat md.c
#include <ruby.h>

VALUE foo(VALUE self)
{
  rb_p(rb_method_data());
}

void Init_md()
{
  rb_define_method_with_data(rb_mKernel, "foo", foo, 0, INT2NUM(42));
}

[pbrannan@zaphod md]$ ruby1.9 -rmd -e foo
42

Paul

Index: include/ruby/ruby.h
===================================================================
--- include/ruby/ruby.h	(revision 15196)
+++ include/ruby/ruby.h	(working copy)
@@ -700,9 +700,12 @@
 
 #define RUBY_METHOD_FUNC(func) ((VALUE (*)(ANYARGS))func)
 void rb_define_method(VALUE,const char*,VALUE(*)(ANYARGS),int);
+void rb_define_method_with_data(VALUE,const char*,VALUE(*)(ANYARGS),int,VALUE);
 void rb_define_module_function(VALUE,const char*,VALUE(*)(ANYARGS),int);
 void rb_define_global_function(const char*,VALUE(*)(ANYARGS),int);
 
+VALUE rb_method_data(void);
+
 void rb_undef_method(VALUE,const char*);
 void rb_define_alias(VALUE,const char*,const char*);
 void rb_define_attr(VALUE,const char*,int,int);
Index: vm_core.h
===================================================================
--- vm_core.h	(revision 15196)
+++ vm_core.h	(working copy)
@@ -326,6 +326,7 @@
     VALUE method_class;         /* cfp[11] saved in special case */
     VALUE prof_time_self;       /* cfp[12] */
     VALUE prof_time_chld;       /* cfp[13] */
+    VALUE method_data;          /* cfp[14] */
 } rb_control_frame_t;
 
 typedef struct {
Index: gc.c
===================================================================
--- gc.c	(revision 15196)
+++ gc.c	(working copy)
@@ -920,6 +920,7 @@
 	  case NODE_FCALL:
 	  case NODE_DEFN:
 	  case NODE_ARGS_AUX:
+	  case NODE_CFUNC:
 	    ptr = (VALUE)obj->as.node.u3.node;
 	    goto again;
 
@@ -985,7 +986,6 @@
 
 	  case NODE_ZARRAY:	/* - */
 	  case NODE_ZSUPER:
-	  case NODE_CFUNC:
 	  case NODE_VCALL:
 	  case NODE_GVAR:
 	  case NODE_LVAR:
Index: class.c
===================================================================
--- class.c	(revision 15196)
+++ class.c	(working copy)
@@ -13,6 +13,7 @@
 #include "ruby/signal.h"
 #include "ruby/node.h"
 #include "ruby/st.h"
+#include "vm_core.h"
 #include <ctype.h>
 
 extern st_table *rb_class_tbl;
@@ -767,6 +768,19 @@
 }
 
 void
+rb_define_method_with_data(VALUE klass, char const * name, VALUE (*func)(ANYARGS), int argc, VALUE data)
+{
+    rb_add_method(klass, rb_intern(name), NEW_NODE(NODE_CFUNC,func,argc,data), NOEX_PUBLIC);
+}
+
+VALUE
+rb_method_data(void)
+{
+  rb_control_frame_t *cfp = ruby_current_thread->cfp;
+  return cfp->method_data;
+}
+
+void
 rb_define_protected_method(VALUE klass, const char *name, VALUE (*func)(ANYARGS), int argc)
 {
     rb_add_method(klass, rb_intern(name), NEW_CFUNC(func, argc), NOEX_PROTECTED);
Index: vm.c
===================================================================
--- vm.c	(revision 15196)
+++ vm.c	(working copy)
@@ -453,6 +453,7 @@
 
 	    cfp->method_id = id;
 	    cfp->method_class = klass;
+	    cfp->method_data = body->nd_cval;
 
 	    val = call_cfunc(body->nd_cfnc, recv, body->nd_argc, argc, argv);
 
Index: vm_insnhelper.c
===================================================================
--- vm_insnhelper.c	(revision 15196)
+++ vm_insnhelper.c	(working copy)
@@ -366,6 +366,7 @@
 
 	cfp->method_id = id;
 	cfp->method_class = klass;
+	cfp->method_data = mn->nd_cval;
 
 	reg_cfp->sp -= num + 1;