Not that anyone else would want to do this, but, it might be useful to someone. I solved this by just adding a blank webform in my DotNetNuke website. I'm sure someone has already done this since its so simple but I couldn't find any posts about it. This is about as simple, and insecure, as it gets, anyone else should add some sort of safeguards to keep someone from randomly generating accounts, but this will accomplish registering from another page. So far this basic approach is working fine for me, but that doesn't mean I'm not ignoring something important, so use with caution:
In the webform's Page_Load event, call this sub:
Private Sub AddCrossSiteUser()
Try
Dim UserCtl As New UserController
Dim User As New UserInfo
User = UserCtl.GetUserByUsername(0, "defaultUser")
User.Profile.FirstName = Request.Form("FName")
User.Profile.LastName = Request.Form("LName")
User.Membership.Username = Request.Form("UserID")
User.Membership.Password = Request.Form("Password")
User.Membership.Approved = True
User.Membership.Email = Request.Form("Email")
User.UserID = UserCtl.AddUser(User)
Response.Write(User.UserID)
Catch ex As Exception
Response.Write("-1")
End Try
End Sub