[matabyte]

Vale, ya lo he conseguido, pongo la solución por si alguien necesita también algo parecido:

La estructura que contiene la lista de directorios es:

Código Delphi [-]
 type
  Tdirectorio = record
    nombre: widestring;
    path:widestring;
    anterior: longword;
    path_completo: widestring;
  end;

var
iso_for:record
             tabla_path:array of Tdirectorio;
             tabla_path_num:longword;
             num_directoriosANSI: longword;
             directoriosANSI: array of Tdirectorio;
          end;

Y la solución para extraterlo:

Código Delphi [-]
     ISO_for.tabla_path_num:=1;
     setlength(ISO_for.tabla_path,ISO_for.tabla_path_num);
     ISO_for.tabla_path[ISO_for.tabla_path_num-1].anterior:=1;
     ISO_for.tabla_path[ISO_for.tabla_path_num-1].path_completo:='\';
     ISO_for.tabla_path[ISO_for.tabla_path_num-1].nombre:='\';

      for a := 0 to iso_for.num_directoriosANSI - 1 do
       begin
         st:=iso_for.directoriosANSI[a].nombre;
         if st[1]<>'\' then st:='\'+st;
         st2:='';
         for b := length(st) downto 1 do
          begin
            if (st[b]='\') then
             begin
               st2:=Copy(st, 0, b);
               result:=false;
               for c := 0 to ISO_for.tabla_path_num - 1 do
                begin
                  if uppercase(st2)=uppercase(ISO_for.tabla_path[c].path_completo) then
                   begin
                     result:=true;
                     break;
                   end;
                end;
                  if result then
                  else
                   begin
                     inc(ISO_for.tabla_path_num);
                     setlength(ISO_for.tabla_path,ISO_for.tabla_path_num);
                     ISO_for.tabla_path[ISO_for.tabla_path_num-1].path_completo:=st2;
                     for c := length(st2) downto 0 do
                      begin
                        if ((st2[c]='\')and(c<>Length(st2)))or(c=0) then
                         begin
                           if c=0 then
                            begin
                             ISO_for.tabla_path[ISO_for.tabla_path_num-1].anterior:=1;
                             ISO_for.tabla_path[ISO_for.tabla_path_num-1].nombre:=copy(st2,c,Length(st2));
                            end
                           else
                            begin
                             ISO_for.tabla_path[ISO_for.tabla_path_num-1].nombre:=copy(st2,c+1,Length(st2));
                            end;
                           break;
                         end;
                      end;
                   end;
               st2:=st2+'\';
             end;
          end;
       end;


      for b := 1 to ISO_for.tabla_path_num - 1 do
       begin
         //Caculamos el hijo
         st2:=iso_for.tabla_path[b].path_completo;
                     for c := length(st2) downto 0 do
                      begin
                        if ((st2[c]='\')and(c<>Length(st2)))or(c=0) then
                         begin
                           if c=0 then
                            begin
                              iso_for.tabla_path[b].anterior:=1;
                            end
                           else
                            begin
                              st2:=Copy(st2,0,c);
                              for a := 0 to ISO_for.tabla_path_num - 1 do
                               begin
                                 if UpperCase(st2)=UpperCase(iso_for.tabla_path[a].path_completo) then
                                  begin
                                    iso_for.tabla_path[b].anterior:=a;
                                    Break;
                                  end;
                               end;
                            end;
                           break;
                         end;
                      end;
//         richedit1.Lines.Add(iso_for.tabla_path[iso_for.tabla_path[b].anterior].nombre+'  --  '+iso_for.tabla_path[b].nombre);
       end;