The Chase 241 for some time wouldn't let you just book a one way, but this recently changed and you can book a one way online. I would book the entire one ay routing online LAX-LHR-DUB. Then call and add the inbound when they come available. See:
http://www.flyertalk.com/forum/briti...way-241-a.html
http://www.flyertalk.com/forum/briti...1-one-way.html
http://www.flyertalk.com/forum/briti...n-voucher.html
http://www.flyertalk.com/forum/briti...eparately.html
You could also try calling and booking the outbound, the agent usually can make a note that the inbound will be added when it comes available you will have to call. Usually just one phone fee.