Sign-up Password plugin

I think this is a great plugin. Has been a pleasure finding it. I personally can't stand auto generated passwords..

I have a question (it is very important)

Is there a way to add "Invitation code" field to this plugin, just as a regular register-plus enables us to do it.

I found that without invitation code the sites are simply super prone to sploggers.. when I use invitation code I

1. can send the code via email & "build a mailing list"

2. prevent a lot of sploggers from registering accounts...

3. make it quite hard for spam bots of various sorts to actually create a blog

If anything, this would be one thing I would like to have...