cranial osteopathy springs to mind - my MIL said our osteopath does a little manipulation to release something that eases her catarrh
ask around for a recommendation (would recommend mine if I knew you lived near, pm me if you want)
avoid citrus fruits, they can trigger the problem
dairy - may or may not be the problem, your consumption may or may not effect her, could certainly try!
and steam inhalation could be tricky, depending on what causes the mucus, the steam might make things worse (think pasta swelling up when cooked)
dry heat might be better - not sure how that applies to a baby.
start a food/event diary, so you can see what is happening actually, you might see a pattern emerge or some other links
I used to take echinacea drops in most of my pregnancies, after asking gp's opinion, she said she's not against it, but can't suggest it either.
I could take nothing else, so I went for it. no problems.
so I figured if it's "ok" while pg, I'll try and give some to DD.
gave her 1/4 of a kid's echinacea tablet twice a day when she had a really bad cold, mashed it up then gave it on a spoon.
it seemed to help her, so not regretting it!
I use the A. Vogel brand.