This probably should be in the anti-splog plugin support area, but I don't seem to see that listed.
I am not seeing how to use this feature with the option of forcing https. That means when using this feature, we are asking users to enter an email/username/password over an non-secure system.
I am thinking this could be qualified as a bug...and a very necessary feature that shouldn't be too hard to implement rapidly and possibly qualifies as a security risk.
I think the fix should be a checkbox stating "force https" positioned below the "Move wp-signup.php " checkbox in the "Rename wp-signup.php" area of the anti-splog plugin.