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.