Miss +[NSObject poseAsClass:]? Don't use this, you might hurt yourself or someone you love.

Poser.m

#import <objc/runtime.h>
#import <stdlib.h>

// declare some of the Objective-C runtime's private parts where we can see them
typedef struct _NXMapTable NXMapTable;
extern NXMapTable *gdb_objc_realized_classes;
extern void *NXMapInsert(NXMapTable *table, const void *key, const void *value);

@implementation NSObject (Poser)

+ (void)poseAsClass:(Class)posedClass
{
	int n, i;
	Class *subclasses;
	
	// refuse to pose as non-superclass
	// XXX a serious implementation should probably raise an error here
	if ( class_getSuperclass(self) != posedClass ) return;
	
	// grab the list of all registered classes
	n = objc_getClassList(NULL, 0);
	subclasses = alloca(n * sizeof(Class));
	objc_getClassList(subclasses, n);
	
	// iterate over the entire class list and re-super all subclasses of posedClass except for this one
	for ( i = 0; i < n; ++i )
		if ( class_getSuperclass(subclasses[i]) == posedClass && subclasses[i] != self )
			// this is a really terrible idea
			class_setSuperclass(subclasses[i], self);
	
	// good lord, we're committing identity fraud
	NXMapInsert(gdb_objc_realized_classes, class_getName(posedClass), self);
}

@end

Come to think of it, one might wonder why I even bother checking the superclass when this entire thing is pretty much the worst idea ever.