You can put the pointers in an NSValue. What key are you going to use?
// Save the TBXMLElement*
NSMutableDictionary *dict = [NSMutableDictionary dictionary];
[dict setValue:[NSValue valueWithPointer:element] forKey:@"whatKey"];
…
// Get the TBXMLElement*
TBXMLElement *el = (TBXMLElement *)[[dict valueForKey:@"whatKey"] pointerValue];