Best way to set extended fields using a form in the manager

I need my manager users to be able to add certain fields to a user profile - ie use something other than the clunky and horrible extended fields interface. I'm slowly tracing my way through the code but I'm getting a little lost.

I'm using 2.2.0pl2

I've made a custom manager theme dir. I've copied across /manager/templates/security/user/create.tpl
and /manager/controllers/security/user/create.class.php to respective custom directories.

I've altered create.class.php to call a different widget in the manager. I haven't changed much there yet - but I can see that I am at least in the right area.

What I can't - for the life of me - work out is where actual 'create the user object' bit of script is. What am I missing?

Because it is a system event... D'oh! So all I needed was a plugin to fire on that event. Not convinced it wouldn't have been better to write a Component to handle this instead - reusing the Login package for manager use. Not sure... Thoughts?