WishList member is more of a membership plugin.I think what Texas Guns is looking for is more of a "licensing system" for plugins. I've been looking for that for quite a while myself but did not find anything very useful...
If there are any hints, they would be highly appreciated!
As far as I am aware, any registration scheme you use would require some part of the script to be encrypted. I don't think I'm alone in that I am wary of running any code I cannot examine.
Yes you will have pirating, but if your plugin is good, you will sell it to enough honest people to make it worth your while.