Nevermind. I'm an idiot. On the f-droid download page, there is a button to download an actual f-droid apk and then a little further down a text link to get the launcher apk. I was trying to install the f-droid apk. I noticed this after I installed nova launcher and wondered why the f-droid icon was in my app drawer. Two mysteries solved.
Anyway, relaunchx seems to be working on Nook GL3.