Maybe airline dependent, but my wife and I lived in Australia before flying to get married in Ireland BNE-SIN-LHR (SQ)-DUB(BD) and she was allowed to carry it on no problem and they layed it out for her on both airlines in the upper storage (full flat!).
Not sure what the policy is for SQ, BD or AC, but you could also have some luck with the FAs on the day ...
Congrats!