Pursuant to that previous post, it seems the solution for text message alerts was staring right at me the whole time!
SPB Phone Suite, nestled within its settings menu, has an option to enable reminder notifications for unread text messages (or missed calls or unread emails if that's more to your liking). What's even neater is that it operates outside of the regular Windows Mobile notifications, so your initial "new message" notification can be one thing while the reminder can be something completely different (probably something a lot louder than the first notification, in my case).