A little bit disgusting, but you can totally create classes with ivars in cocoa script :D

ObjCClass.cocoascript.js

// Copy-paste this into 💎Sketch.app and run it 🔥
// Scroll to bottom for usage

// Use any C function, not just ones with BridgeSupport
function CFunc(name, args, retVal) {
  // Due to particularities of the JS bridge, we can't call into MOBridgeSupport objects directly
  // But, we can ask key value coding to do the dirty work for us ;)
  function setKeys(o, d) {
    const funcDict = NSMutableDictionary.dictionary()
    funcDict.o = o
    Object.keys(d).map( k => funcDict.setValue_forKeyPath(d[k], "o." + k) )
  }
  function makeArgument(a) {
    if (!a) return null;
    const arg = MOBridgeSupportArgument.alloc().init();
    setKeys(arg, {
     type64: a.type,
    });
    return arg;
  }
  const func = MOBridgeSupportFunction.alloc().init();
  const argArr = args.map( makeArgument );
  setKeys(func, {
    name: name,
    arguments: argArr,
    returnValue: makeArgument(retVal),
  })
  return func;
}

/*
@encode(char*) = "*"
@encode(id) = "@"
@encode(Class) = "#"
@encode(void*) = "^v"
@encode(CGRect) = "{CGRect={CGPoint=dd}{CGSize=dd}}"
*/

// This assumes the ivar is an object type. Return value is pretty useless.
const object_getInstanceVariable = CFunc("object_getInstanceVariable", [{type: "@"}, {type:'*'}, {type: "^@"}], {type: "^{objc_ivar=}"});
// Again, ivar is of object type
const object_setInstanceVariable = CFunc("object_setInstanceVariable", [{type: "@"}, {type:'*'}, {type: "@"}], {type: "^{objc_ivar=}"});

function ObjCClass (handlers, ivars = [], superclass = NSObject){
  var className = "GeneratedClass" + NSUUID.UUID().UUIDString()
  var cls = MOClassDescription.allocateDescriptionForClassWithName_superclass_(className, superclass)
  // Add each handler to the clas description
  for(var selectorString in handlers) {
    var selector = NSSelectorFromString(selectorString)
    cls.addInstanceMethodWithSelector_function_(selector, handlers[selector])
  }

  ivars.map( name => cls.addInstanceVariableWithName_typeEncoding(name, "@"))

  return cls.registerClass();
};

function getIvar(obj, name) {
  const retPtr = MOPointer.alloc().init();
  object_getInstanceVariable(obj, 'ivar', retPtr);
  return retPtr.value();
}


/**************************************
************** Usage ******************
***************************************/


const MyClass = new ObjCClass({
  test() {
    log("ivar is " + getIvar(this, 'ivar'));
  },
}, ["ivar"], NSObject)


const obj = MyClass.new();

obj.test();

// Yes you can do this
obj.setValue_forKey("abcd", "ivar")
obj.test();

// But this is more elegant!
object_setInstanceVariable(obj, "ivar", "efgh");
obj.test();

/* output:
ivar is null
ivar is abcd
ivar is efgh

Script executed in 0.025665s
*/