Use hook_form_FORMID_alter

It'd be nice to break up email_registration_form_alter into four separate hook_form_form_id_alter functions, rather than one big hoom_form_alter function. For one thing it'll improve performance a bit; for another thing, it's that much easier to allow other modules to override some of the form behavior (notably custom_email_registration_name_submit, which sets the post-login redirect to something that might not be desirable.)