The simple solution, I think, is to give permission. Look for ways to optimize, simplify, refactor. Do it within story work when that makes sense. Don't "hide" it but if you have some time, do what you feel is important and interesting let the team know what you found out. Set the example. You shouldn't need a JIRA ticket to improve the system, do a quick spike or take a s** and on my team, you don't.
Yes, at some point, management can ask you to justify the work you did. They are paying you, after all. Scrum and agile was never about letting people do whatever they wanted do. Management has already had a role in making sure there were appropriate limits set. In other words, if you're responsible for building a word processor and end up building a video game, I think it's appropriate for someone to ask what are you doing?