I would say no, don't use the plural. English adjectives don't have plurals. If we continued to inflect whole phrases as if in the language they came from, we'd say 'courts martials'; but we say 'courts martial' (if adhering to French style - or of course 'court martials' if wholly Englishing it).
The principle is probably that English has a category of plurality in nouns, and we admit foreign ways of forming it. But we don't ever import foreign grammar as such.
For example, you don't run 'emeritus' through the Latin cases, and say 'Oh, Professor Emerite', and 'give it to the professor emerito'. Adjectival plural is in the same boat: it's a feature of Latin grammar but not of English.
It would be different if the whole phrase was Latin: and indeed professor emeritus could be considered so, but its plural would then be professores emeriti.
