My DD is now 2 and breastfed, though some days she doesnt feed at all, other days she feeds a few times - its up to her really! She is allergic to CMP, eggs and nuts.
My DD was a very "sicky" baby in the early days. After a breastfeed she would be really sick - it seemed liked gallons of milk came back up. H/V's and GP's were not very worried, as my DD was otherwise a happy baby, who was gaining weight and generally thriving. However, I realised that every time I had cereal with milk, the next morning my DD would be very sick after a breastfeed. I decided to cut out raw dairy and it made a huge difference. Pretty much no more sick within a few days, unless I ate too much raw dairy.
When my DD was around 1 she developed eczema around her mouth. It has never been horrific, just very itchy for her at times, and has always cleared up quickly with antihistamine creams. I'm pretty sure it was linked to my diet and the allergens I was eating. My DD had a slight red patch on her chin for roughly 9 months - it wasnt always visible unless she lifted her chin, but nothing took it away (I didnt totally eliminate allergens though).
Around christmas last year my DD's eczema almost vanished...around a time when she was breastfeeding all the time and I was eating a ridiculous amount of chocolate, so I am not sure why it improved...I secretly hope its a sign that her allergies have improved. I still moisturise her face every day at least once or twice and use a/h cream as soon as any redness appears but her skin is looking fantastic!
Anyway, sorry for long post - we have a slightly different situation in that we know what my dd's allergens are, but yes, you are right, it can be difficult when breastfeeding, though in many respects it can be easier as you know that your baby is getting all the benefits of breastfeeding rather than having to try differnt formulas which are not always particularly nice.
The advice about eliminating allergens from a mother diet varies - our consultant advised me to continue as I had been - i.e avoiding alot of raw dairy (we discovered my DD's allergies after starting to wean, were referred to a consultanr, but by this point I had avoided raw dary for months anyway due to realising myself that it was affecting my DD) but not completely eliminating them from my diet as my DD seemed fine with this.
Hope you get more answers soon.