I personally think that it was a lovely idea and I would have no problem with it.
When my DS2 was born I lost a lot of blood and was semi-conscious for nearly 2 hours. During this time DS2 stayed skin to skin with DH and spent about an hour suckling on him. From my point of view at the time I was desperate to do this myself but was too ill. Fortunately having DH there meant that he had everything he needed other than milk (which I supplied shortly afterwards) but he was content and cuddled during that time. The MWs didn't bat an eyelid.
My DS doesn't have a dummy (he rejected it, I don't have a problem with them) as he prefers to comfort himself on my breasts. He does this far less than DS1 sucked on his dummy - it's not a problem. Having someone else offer their breast as a comfort would be something he'd really like, I'm sure.
The only problem with someone else BFing your baby is that some diseases can be passed through milk. As long as that's not a problem, then shared BFing seems logical to me. Taking that a step further, why can't another breast be used as a comforter when there's no milk to transfer and no disease risk?
The ickyness comes, I think, with the sexualisation of breasts which in itself is fine (mine are important to our sexual relationship - I do not take that away from them) but they are absolutely dual-functional. For DS, they are food and comfort. For me and DH they are part of our own personal relationship. There is therefore nothing sexual (obviously, which you've made clear) about what your mother did - she was just trying to comfort your little one in a way she understands. That, to me, takes the ickyness away.