I have an Apex method that is marked as a remoteAction which is attempting to take two parameters: a Lead and a List of custom objects ("Custom_Object__c" isn't the real object's name, just changed to protect the innocent);

@RemoteAction
public static Boolean insertData(Lead l, List<Custom_Object__c> customObjects) {
   // insert the Lead
   insert l;
   // assign the lead Id to each custom object in the list
   for(Custom_Object__c c : customObjects) {
      c.Lead__c = l.Id
   }
   // insert the custom objects
   insert customObjects;
   // return true (just for example)
   return true;
}

 Over in Javascript, I'm calling this remoteAction with the following data:

// sample data
var lead = {
   FirstName = 'Test',
   LastName = 'Test',
   Company = 'Test',
   Email = 'test@test.com',
   Phone = '555 555 1212'
};

var customObjects = [
   {
      Custom_Field1__c = 'A',
      Custom_Field2__c = 'B'
   },
   {
      Custom_Field1__c = 'A',
      Custom_Field2__c = 'B'
   }
];

MyApexClass.insertData(lead, customObjects, function(result, event) {
   // this would be the callback mojo
});

This code will make it as far as the insertData method, but I get the following error:

Visualforce Remoting Exception: undefined

I tried changing the insertData method to take the second parameters as List<sObject> instead of List<Custom_Object__c> (and modded the code to use dynamic DML to support this), but then I get this error:

Visualforce Remoting Exception: Unable to determine SObject type: SObject.  Please provide an 'id' or 'sobjectType' value.

I interpret this error as needing to supply a 'sobjectType' property to either a) the elements in my "customObjects" array or b) the array itself. I've tried all of the following permutations but still get the same error:

// ... add the property to the element doesn't work
var customObjects = [
   {
      Custom_Field1__c = 'A',
      Custom_Field2__c = 'B',
      sobjectType = 'Custom_Object__c'
   },
   {
      Custom_Field1__c = 'A',
      Custom_Field2__c = 'B',
      sobjectType = 'Custom_Object__c'
   }
];

// add the property to the array doesn't work
customObjects.sobjectType = 'Custom_Object__c';

// for fun, tried to indicate that it's a list, these don't work either
customObjects.sobjectType = 'List<Custom_Object__c>';
customObjects.sobjectType = 'Custom_Object__c[]';

Can someone point me in the right direction here? Is there another way to specify the sobjectType property? Are there issues with multiple params on a remoteaction, or issues with collection-based params?