How to pass the popup blocker
Well I finally found the solution, and it works pretty well. What I tested is to use in the Login page a link button instead of a normal button.
In the click event, after the user name and password is validated, I register a client side script to open the window.
In Javascript, the trick here is to declare the window, not just open it.
It seems that a Link button fire a slightly different Click event than a Button so it is not considered as a possible popup by Google, so that's why it works here.
That I suppose is an explanation, but if you know better ;-)