I got a simple way to extend a midi item and keep the loop property.
Custom: Extend Midi Item to Time Selection
Insert or extend MIDI items to fill time selection
Item: Select item under mouse cursor
Item properties: Loop item source
If you select the item and want to run the action regardless of mouse location you can also do this:
Custom: Extend Midi Item to Time Selection
SWS: Save selected item(s)
Insert or extend MIDI items to fill time selection
SWS: Restore saved selected item(s)
Item properties: Loop item source