Azure Keyvault add Function MSI via ARM

I think Managed Service Identity is a great concept and I love keyvault. However:

When I use the script using an incremental resource group deployment:

Sample is modified for brevity

{
      "type": "Microsoft.KeyVault/vaults",
      "name": "[parameters('keyvaultName')]",
      "apiVersion": "2015-06-01",
      "properties": {            
        "accessPolicies": [
          {
            "objectId": "[reference(parameters('functionAppName'), '2016-08-01', 'Full').identity.principalId]",
            "permissions": {
              "keys": [],
              "secrets": [
                "Get"
              ]
            }
          }
        ]
      },
      "dependsOn": [
        "[resourceId('Microsoft.Web/sites', parameters('functionAppName'))]"
      ]
    },
    {
      "apiVersion": "2016-08-01",
      "type": "Microsoft.Web/sites",
      "name": "[parameters('functionAppName')]",
      "kind": "functionapp",
      "identity": {
        "type": "SystemAssigned"
      },
    }

It deploys successfully and adds the MSI to keyvault, but --

It blows away the already assigned access policies. Is it possible for arm to preserve accessPolicies and only add/update policies that match?

Without this it's impossible to fully script a deployment with a MSI and also assign the principal to keyvault..

Am I missing something?

As the author of the blog post, I'll post the details per the mods:

When you deploy a resource of type Microsoft.KeyVault/vaults/accessPolicies with the name “add”, it will merge in your changes. This special child resource type was created to allow Managed Service Identity scenarios where you don’t know the identity of a VM until the VM is deployed and you want to give that identity access to the vault during deployment.

An incremental deployment can be used along with this json to achieve the objective:

{
    "$schema": "http://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {
        "vaultName": {
            "type": "string"
        }
    },
    "resources": [
        {
            "type": "Microsoft.KeyVault/vaults/accessPolicies",
            "name": "[concat(parameters('vaultName'), '/add')]",
            "apiVersion": "2016-10-01",
            "properties": {
                "accessPolicies": [
                    {
                        "tenantId": "dfe47ca8-acfc-4539-9519-7d195a9e79e4",
                        "objectId": "5abe9358-10ae-4195-ba23-d34111430329",
                        "permissions": {
                            "keys": ["all"],
                            "secrets": ["all"],
                            "certificates": ["all"],
                            "storage": ["all"]
                        }
                    }
                ]
            }
        }
    ],
    "outputs": {
    }
}