The idea was correct, just the port was wrong. PIND is to be used instead of PORTD, and it makes sense: it's the input port register, from which the T0 value is taken.

$repeat 1000
   PIND=0xFF // Set PORTD high
   #800000 // wait 50ms
   PIND=0x00 // Set PORTD low
   #800000 // wait 50ms
$endrep