Is it possible to extract(as xml) a Maintenance Unit and it's components using extensibility rule?

You will have to go one by one and get everything under that maintenance unit. All items will get you all the maintenance units. You can then probably edit the XML and get what you want. But if it is just a backup, I would go with all options and back up all the maintenance units.

Does it have to be a BR? This is exactly what XFProject files solve: the export and import of full maintenance units with as little as 5 lines of xml.

RobbSalzmann​ , ckattookaran​ , db_pdx​ - Thank you all for your valuable responses.

Robb - I wrote the below code to extract entire maintenance unit without specifying each objects and it is working as expected. Here, I made "isExtractAllItems = True". The code isolates the desired maintenance unit from the extracted XML, reconstructs a minimal XML structure containing just this unit (and its workspace context), and saves the result as a new XML file for backup or migration purposes. And both the methods were tested and both works as expected. 

Thanks everyone for the inputs!
Cheers!

Case Is = ExtenderFunctionType.Unknown, ExtenderFunctionType.ExecuteDataMgmtBusinessRuleStep
					
						
						'Get Configuration Settings
							Dim configSettings As AppServerConfigSettings = AppServerConfig.GetSettings(si)
							'Data Management extract location
							Dim folderPath As String = FileShareFolderHelper.GetDataManagementExportUsernameFolderForApp(si, True, configSettings.FileShareRootFolder,
							 si.AppToken.AppName
							 ) & "\" & DateTime.UtcNow.ToString("yyyyMMdd") & "\Extracts"  'If the directory does not exist create
						If Not Directory.Exists(folderPath) Then Directory.CreateDirectory(folderPath)
							'Full path and file name for extract
							Dim filePath As String = folderPath & "\Dashboards " & DateTime.UtcNow.ToString("yyyyMMdd") & ".xml"
						'If file already exist
						If File.Exists(filePath) Then File.Delete(filePath)
						
							Dim isExtractAllItemsTrue = True
							Dim isExtractUniqueIdsTrue = True 
							Dim xmlOptions As New XmlExtractOptions(isExtractAllItemsTrue, isExtractUniqueIdsTrue) 
							Dim xmlString As String = String.Empty
							Dim maintUnitToExtract As String = "YourMaintenanceUnit"							
							
							
							' extract specified items doesn't seem to work, but we need it
							Dim extractDict As New Dictionary(Of XmlExtractItemPk, Boolean) From {
								{New XmlExtractItemPk(XmlExtractItemType.DashboardMaintUnit, maintUnitToExtract), True}
																								}	
							
							'Execute the Metadata Extract
							Using dbConnFW As DBConnInfo = BRAPi.Database.CreateFrameworkDbConnInfo(si)
								Using dbConnApp As DBConnInfo = BRAPi.Database.CreateApplicationDbConnInfo(si)
									xmlString = XmlExtractController.ExtractXML(dbConnFW, dbConnApp, Nothing, xmlOptions, extractDict, XmlLoadExtractType.ApplicationWorkspaces)
								End Using
							End Using	
							
							Dim xmlDoc As XDocument = XDocument.Parse(xmlString)
							Dim extractedMaintUnitElement As XElement = xmlDoc.Descendants("maintenanceUnit").Where(Function(mu) mu.Attribute("name").Value = maintUnitToExtract).FirstOrDefault()
							
							If extractedMaintUnitElement IsNot Nothing Then
							    Dim workspaceElement = extractedMaintUnitElement.Ancestors("workspace").FirstOrDefault()
							    Dim newWorkspaceElement As New XElement("workspace", workspaceElement.Attributes())
							    newWorkspaceElement.Add(New XElement("maintenanceUnits", extractedMaintUnitElement))
						
							    Dim newXmlDoc As New XDocument(
							        New XElement("OneStreamXF", xmlDoc.Root.Attributes(),
							            New XElement("applicationWorkspacesRoot",
							            	New XElement("workspaces", newWorkspaceElement)
							            )
							        )
							    )
							    newXmlDoc.Save(filePath)
							
							End If		
				End Select