Unable to use reference() inside Microsoft.Web/sites/{site}/config/web properties

I am trying to set `alwaysOn` for a web app based on whether the app service plan is in a tier that supports it. The `reference()` call below fails when it is inside the `properties` (see below). If I move this `reference()` call into an output parameter it succeeds. Am I missing something or doing something wrong? Thanks!

``` javascript
{
"$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
"contentVersion": "1.0.0",
"parameters": {
"name": {
"type": "string",
"metadata": {
"description": "name of the resource"
}
},
"location": {
"type": "string",
"defaultValue": "[resourceGroup().location]",
"metadata": {
"description": "location of the resource"
}
},
"appServicePlan": {
"type": "string",
"metadata": {
"description": "app service plan to create the web app on"
}
}
},
"variables": {
"apiVersion": "2016-03-01",
"convertToAlwaysOn": {
"F": false,
"B": false,
"S": true,
"P": true
}
},
"resources": [
{
"apiVersion": "[variables('apiVersion')]",
"type": "Microsoft.Web/sites",
"name": "[parameters('name')]",
"location": "[parameters('location')]",
"properties": {
"name": "[parameters('name')]",
"serverFarmId": "[parameters('appServicePlan')]"
},
"resources": [
{
"apiVersion": "[variables('apiVersion')]",
"type": "config",
"name": "web",
"dependsOn": ["[resourceId('Microsoft.Web/sites', parameters('name'))]"],
"properties": {
"alwaysOn": "[variables('convertToAlwaysOn')[toUpper(substring(reference(resourceId('Microsoft.Web/sites', parameters('name'))).sku, 0, 1))]]"
}
}
]
}
]
}
```

Reproduce the supplied deployment template in Azure, starting with the nested Microsoft.Web/sites/config/web resource and its reference() expression. Compare evaluation inside properties with the output case described in the issue. Done means documenting whether this usage is supported and, if not, recording the supported limitation or required template adjustment.

Written by the indexing model from the issue text.