As an ORM, Aurelius abstracts the underlying database and allows the user to work with the objects. So it doesn't make much sense to use the "low-level" column names as if they were properties of the object. Conceptually, you are querying objects, not tables.
But you can achieve what you want this way:
Find<TEntityCustFld>
.Where(Linq.Sql<Integer>('A.MainId = ?'), someValue)
.UniqueResult;