How to approve script snippets from a jenkinsfile via the groovy script console?

You can use ScriptApproval#approveSignature method. Here is an example that works on my Jenkins 2.85

def signature = 'new groovy.json.JsonSlurperClassic'
org.jenkinsci.plugins.scriptsecurity.scripts.ScriptApproval.get().approveSignature(signature)

import org.jenkinsci.plugins.scriptsecurity.scripts.*
toApprove = ScriptApproval.get().getPendingScripts().collect()
toApprove.each {pending -> ScriptApproval.get().approveScript(pending.getHash())}