Can an adverb be a predicate?

Solution 1:

Adverbs could be part of a predicate, to be more exact, a subjective complement, but it doesn't happen very often and they are used either idiomatically or in a sentence where a participle or part of the sentence is elided.

In your quote, "Life is once" could be considered as short for

Life is given to people once. Life is lived once by people.

and it doesn't necessarily mean once is a subjective complement alone once elided words are placed. The past participle given or lived is elided in your example.

He will be (coming) back in 5 minutes: coming is elided.

He is out (of the house/building/restaurant...): a place is elided.

The secret is out: the secret has been discovered/revealed/known.

There are more examples where an adverb could be used as a subjective complement. They should be learned on a case-by-case basis.

Solution 2:

Some adverbs definitely can be predicates: words of location - here, indoors, abroad - are usually seen as adverbs, and obviously commonly used as predicates.

One could make the case that the common construction {event_noun}{be}{time_adverb} (e.g. "the concert was yesterday") also uses adverbs as predicates.
But another view would be that the {be} there is used not as a copula but in the meaning "to happen, take place", in which case it's not using the adverb as a predicate.
Both those views are reasonable, though I'd lean towards the latter.

"Life is once" seems to follow that pattern, so whether once is a predicate in that particular phrase depends on how you choose to analyse the is.

Either way, it's certainly not a common kind of usage of once. I'd go so far as to suggest it was intentionally chosen to grab attention with how unusual it is.

(Terminology note: For sentences like "Jill is tall", I've seen the word predicate sometimes referring just to the "tall" and other times to the whole "is tall". In this answer I'm using the word the same way the OP did.)

Solution 3:

It is not grammatical but it is understandable metaphorical language. It is an advertising slogan where, as with poetry, certain liberties can be taken.

The problem is that Life and is and once do not go together except metaphorically.

Life is an abstraction. That is, not "a life" (a particular life) but the concept that refers to a state of animation, of being alive.

We can use the verb is with abstract concepts, for is means not only exists but can be said to be.

Life is short -> Life can be said to be short. "X is {predicate complement}" means that X has the qualities which {predicate complement} refers to.

once modifies things which occur. We normally say "That happened once".

When we complete the verb is with the adverbial complement once, we treat the verb is as if it were synonymous with the verb happens, which it is not.

By extension, when we treat the verb is as if it means "happens", we treat the subject of the predicate, Life, as if it were not an abstraction but an event: Life happens... but abstractions or concepts cannot "happen".

The predicate subject complement recasts the predicate and the subject into meanings they do not have except metaphorically, by recasting them from their actual grammatical categories into other grammatical categories. The statement violates the rules of grammar but in an intelligible manner.

Here's what happens if we include "Forever" :)

    while(true)
      {
         for i = 1 to 1 {
           var result = (Event)Life
         }
      }