While the rules may state that the Amex 241 may only be used for return tickets many people have reported in this forum that they have used them for oneways. See:
http://www.flyertalk.com/forum/briti...-question.html
http://www.flyertalk.com/forum/briti...ay-trip-2.html
The other alternative for the OP is to just book an available return date (pick any available date or flight around your intended return) and then change the return when it becomes available. No muss, no fuss.