I worked out an alternative registration process using the member service to achieve what I wanted. My register member action method now looks like this:

[HttpPost]
[AllowAnonymous]
public async Task<ActionResult> HandleRegisterMember([Bind(Prefix = "registerModel")]RegisterModel model)
    {

        if (ModelState.IsValid == false)
        {
            return CurrentUmbracoPage();
        }

        var username = model.UsernameIsEmail || model.Username == null ? model.Email : model.Username;
        var memberService = Services.MemberService;

        if (memberService.GetByUsername(username) == null)
        {
            //register member
            var member = memberService.CreateMember(username, model.Email, model.Name, model.MemberTypeAlias);
            memberService.Save(member);
            memberService.SavePassword(member, model.Password);

            //authenticate member
            var user = await UserManager.FindAsync(username, model.Password);
            if (user != null)
            {
                await SignInAsync(user, isPersistent: false);

                //if there is a specified path to redirect to then use it
                if (!model.RedirectUrl.IsNullOrWhiteSpace())
                {
                    return Redirect(model.RedirectUrl);
                }

                //redirect to current page by default
                return RedirectToCurrentUmbracoPage();
            }
        }
        else
        {
            //username already exists
            ModelState.AddModelError("registerModel", "The email address provided has already been registered");
        }

        return CurrentUmbracoPage();
    }