Is it permissible to return the greeting of Salaam inside the lavatory after passing the main door but before entering the private stall, knowing that I am performing ablution?

Al-Khala' (the latrine/toilet) is the place designated for relieving oneself. Therefore, there is no harm in extending or returning the greeting of Salaam, or in saying "Bismillah" (in the name of Allah) at the beginning of ablution in the washing areas, even if they are adjacent to the latrines.