Hi Antonio,
actually you're partly true.
In fact, you can bind a page layout not only to business roles but also to account roles.
This process consists of two parts.
Create two page layouts for accouts, one where all necessary fields are maintainable for prospects and another one where all fields are read-only.
Then go into the assignment of page layouts and choose Business Object "Account" and Instance Type "Role".
There you can maintain the page layouts in a matrix structure consisting of a) business roles and b) account roles.
These accounts roles are in fact related to either customers or prospects.
This can be checked in fine-tuning task "Accounts" --> "Account roles". Each roles is either assigned to customers or prospects.
I can elaborate further if needed but this works definitely as we've have done so in several customer projects.
BR,
Jan