FAQ.How can I set the file comment using AppleEvents?
Feedback.

Answer: Ask the Finder via Apple Events:
Example:

Function writeFileComment(myFile as FolderItem, myCmt as string) As Boolean
  dim ev as AppleEvent
  dim eobj as AppleEventObjectSpecifier
  
  eobj = GetNamedObjectDescriptor("file",nil,myFile.AbsolutePath)
  
  ev = NewAppleEvent("core","setd","MACS")
  
  ev.ObjectSpecifierParam("----") = GetPropertyObjectDescriptor(eobj,"comt")
  ev.StringParam("data") = myCmt
  
  return ev.send
End Function

Notes: The MBS Plugin has a folderitem.comment property for this.