Learn French: Tu peux monter sur scène, ne serait-ce que pour le refrain. - You can get on stage, if only for the chorus.

This is a fixed phrase built using the conditional of c'est with an inverted subject and verb (serait-ce). It is wrapped in the restrictive structure ne ... que, which means "only". Literally, it translates to "were it only". You will never see a pas here because it is a restriction, not a negative statement.

Would I need to change serait-ce to a past tense if the main sentence was in the past?

No. The phrase ne serait-ce que is completely invariable. Even if you changed the beginning to the past tense, like Tu es monté sur scène (You went on stage), you would still use ne serait-ce que to express "if only".

Why is there no article before scène in monter sur scène?

Monter sur scène is a fixed idiom meaning "to go on stage" or "to take the stage" (literally, "to climb on stage", since stages are usually raised). Because it operates as a set phrase, the definite article (la) is dropped, much like how we say "on stage" in English rather than "on the stage".

What kind of grammatical structures can follow ne serait-ce que?

It is very flexible! As seen in this sentence, it can be followed by a prepositional phrase (pour le refrain). It can also be followed directly by an infinitive verb (e.g., ne serait-ce que pour voir), a noun, or a pronoun. It simply introduces the minimum condition or effort you are suggesting.