I broke a d string and in my infinite laziness replaced it with a b (gauge 18) string. I actually love the sound, and would like to keep it in the future, its like halfway to nashville tuning... but will I destroy my guitar's neck in the long run using this setup?
No, you're in no worry of harming your guitar. Because the b string is lighter and you're tuning it to a lower pitch, you're only taking tension off the neck, not putting tension on it, otherwise every time we took all our strings off we'd be in risk of harming our guitars (which we're not). The only problem is when you start using thicker strings than usual (eg using 7 string gauge on a 6 string at standard E tuning). You may run into some intonation issues and a bit of neck bow, but that's very easily adjusted.