Hi Alessandro,
I have an idea but I haven't implemented yet. May be you can try this.
Create a login page under SICF public node. So any thing created under public can be accessible directly. Once user login, redirect from this login page to actual application.
But if you create this login page using SAPUI5 application, you will have to figure out how to create SAPUI5 application under public node in SICF.
I am not sure how much this will help. I am trying to figure out another way. Once I get solution, I will surely let you know and meanwhile if you find a solution please do let me know.
Thanks
Ravi.