Ecgberht, do you have any evidence that anyone else besides you thinks that? I can see it in an old-style nursery song like 'Baa, baa, black sheep,' but otherwise it doesn't seem remotely like modern English.
New Fowler's Modern English Usage, 3rd ed., ed. Burchfield (1996), pp. 317, 193:
-ful. The right plural for such nouns as cupful, handful, mouthful, spoonful, teaspoonful, etc., is cupfuls, etc., not cupsful, etc. See CUPFUL; PLURALS OF NOUNS 9.
cupful. 'A cupful is a measure, and so three cupfuls is a quantity regarded in terms of a cup; three cups full denotes the actual cups, as in three cups full of water' (COD, 1990).