How to iterate on all session belonging to the same user, or all tabs?

You must create a session group for each user, then iterate on all the sessions from this group, and possibly on all client processes for each session:

(* We get the session group state for this user: *)
  let state =
    Eliom_state.Ext.volatile_data_group_state
      ~scope:Eliom_common.default_group_scope (Int64.to_string user_id)
  in

  (* We iterate on all sessions from the group: *)
  Eliom_state.Ext.iter_volatile_sub_states
    ~state
    (fun state ->
      (* We iterate on all client process states in the session: *)
      Eliom_state.Ext.iter_volatile_sub_states
        ~state
        (fun state ->
          let a = Eliom_reference.Volatile.Ext.get state my_ref in
          ...)))