I was b'maid for my friend when DS was 9 weeks old. Yes, weeks. Big huge mistake for many reasons, but I got through it.
The dress was red satin, and I ordered a reversible hotsling with gold brocade on one side. It was *gorgeous* with the dress.
But I didn't wear him until the reception.
If i were in your shoes, I'd have DH bow out. Then he can stand to the side and hold Ez, and you can do your duties; if hubby is just a groomsman, it's a lot less of a deal than a MOH bowing out, know what I mean?
In our situation, the bride ordered us to be there at 1pm. The ceremony started at 6. ggggggggggaaaaaaaaaaaaaaaaaaacccccccckkkkkkkkkkkkk kk.
Hubby and DS hung out in the church's "cry room" all day, coming to find me when milk was needed. I'd go into the cry room, pretty much strip to the waist (bride required us to be dressed *extremely* early and the satin didn't stretch LOL), and nurse DS. Then I'd re-dress and go back. The cry room was soundproof, had a one way window so DH could see (and hear, over speakers) the wedding while watching DS and watching me sway back and forth in the July heat, sun coming down through the massive stained glass windows, heat waves coming off of her massive wedding party (8 or 9 attendants on each side, 2 flowergirls, 2 ringbearers, candle lighters...).....
They went alone to the reception while I went off with them for pictures; I'm sure my boobs got bigger as the pictures went on.
DH checked out the site for me, made friends with some security guards (one of whom had 4 kids who were all breastfed) to make sure I wasn't bothered while feeding DS downstairs, and got us all situated.
It was a bad experience made OK by my hubby.
So I'd suggest trying something like that.
And it's almost guaranteed that months after the wedding, you'll mention all the trouble you went to, and she'll express a complete lack of knowledge of all you went through, and will say "oh you should have told me, I would have let you wear DS!"