Although almost all first-declension nouns are feminine or masculine, there seem to be a handful of adjectives that belong to the first declension for all genders, and at least one substantive noun, Pascha, that could belong to the first declension while being treated as a neuter noun (see my previous question Was "Pascha" ever used as a neuter first-declension noun?).
The "first-declension neuters" don't seem to inflect exactly the same as feminine or masculine nouns of the first declension. For example, when neuter, Pascha seems to follow the rule that nominative and accusative of neuter nouns are always identical, so we have
Nominativo hoc Pascha, genitivo huius Paschae, dativo huic Paschae, accusativo hoc Pascha, vocativo o Pascha, ablativo ab hoc Pascha
(Donatus graecus a, "De Nomine", lines 53-55, p. 271 in Donati Graeci, by Federica Ciccolella (2008))
This passage from Donatus graecus a says that Pascha has no plural forms ("pluralia non habet"). I know that the genitive plural Pascharum has in fact been used in some texts, so at first I assumed that the absence of plural forms in the Donatus graecus was simply because of semantic reasons (Pascha being a proper noun, it would rarely be necessary to pluralize it).
But now that I think on it further, it actually seems to me that in the nom/acc/voc plural, there is no form that would be unproblematic as a candidate for the plural of a first-declension neuter. Almost without exception, neuter plural nouns end in the suffix -a in the nominative, accusative and vocative case (the only exceptions I know of, mentioned in this mailing list post, are historical duals like duo and ambo, and haec and quae; in haec at least the part after a is derived from a separate suffixed element). So the regular plural of an a-stem neuter Pascha would be expected to be Pascha- + -a, which presumably would yield the form Pascha (identical to the singular). Although homophony between singular and plural forms undoubtably exists for some Latin words, it seems odd to me in this situation, and I feel like there might be a tendency to avoid it.
However, I feel very doubtful that a neuter noun could take the -ae-type plural that is found for feminine and masculine nouns of the first declension.
So I'm now more inclined than I was at first to accept Donatus graecus a's statement that plural forms do not exist for the neuter noun (at least, not in the nominative, accusative or vocative cases). And it seems to me that the same would likely apply to the adjectives that are supposed to have been used in the first declension to modify neuter nouns, like advena (mentioned in an answer by fvogel) and verna (see this passage in Beiträge zur Griechischen und Römischen Literaturgeschichte, Volume 2). It doesn't seem to have been too uncommon for Latin adjectives of one ending to be defective in the neuter nom/acc/voc plural; that is allegedly the case for a number of third-declension adjectives (I mention some relevant references in my question here).
Can anyone confirm or disprove my hypothesis that first-declension neuters were defective in the nominative/accusative/vocative plural? If I'm wrong, how were the plurals formed: with -a or -ae?