Good job last night!
To me, so early on, if a "no thanks" such answer "doesn't work" - it is ok to use the kind of lie you had in mind. The important thing- MOST important thing- is for you to get through the event sober. Anything that supports that- even avoiding the groom best you can or "mysteriously" being in the ladies at significant points, whatever, is for your own sober good.
For me, no event is worth my sobriety. And early days, having a plan- always keeping a "drink" in hand is a great one- is crucial.
You can do it.