How can I add warnings to folder level outputs that will display in calculation results?

Answer

The folder levels outputs have the same capabilities. See this example:

quoteProcessor.addOrUpdateOutput(
  "ROOT", 
  ["resultName": "BranchTier",
   "resultLabel": "Branch Tier",
   "result" : branchTier,
   "alertMessage": err,
   "alertType": (err)?"RED":null,
   "userGroup" : "QuoteLineHiddenFields"
  ]