Sorry for this late response. I'm trying to catch up after a prolonged absence from the forum.
I find this subject somewhat amusing. With the exception of David's example of "liquified eggs", which I've never actually seen for sale, but am sure they exist somewhere, I can't imagine "egg" as a non-count noun on a list. If it's a shopping list, you buy "eggs", presumably by the dozen. If it's a recipe, the quantity is going to be more specific. It could be "one egg", but, as Gustavo says, it wouldn't just be "egg".
I like David's example of asking whether a dish "contains any egg (noncount noun)". "Egg" is also used as a non-count noun in the expression "to have egg on one's face", a figurative expression for being embarrassed.
Some years ago, I had a Chinese girlfriend, and while she spoke English well enough to get around, there were a few gaps in her knowledge. One day, when she was cooking dinner for us at my house, she asked me if I had "egger". I answered that, yes, I had eggs. Needless to say, she had egger on her face.
To get back to the ostensible subject of the thread, though, which, as near as I can tell, has not yet been directly addressed, the simple subject of Jiho's examples (a) and (b) is the singular "thing", which requires the singular verb "is". The same is true of Gustavo's examples (a') and (b'). Even though the predicate noun in (b') is the plural "eggs", the subject of the sentence is still singular, and requires a singular verb.