Tamburello is a technical book dealing solely with the cause of Ayrton’s accident, so the topic of the Austrian flag is not discussed there.
According to The Life of Senna book by Tom Rubython, late on Saturday night Senna told his girlfriend Adriane over the phone that “when he won he would uncoil an Austrian flag and fly it on his victory lap in honour of Roland Ratzenberger.”
The same book page 398: “at the hospital it was revealed that nurses had discovered a small furled Austrian flag hidden in the sleeve of Senna’s overalls.”
The Face of Champion CD released in 1995: “before climbing into the car, according to TV commentator Galvao Bueno, Ayrton had asked Julian Jakobi to get him an Austrian flag.”
I remember reading one article where it stated the flag was found in the cockpit; unfortunately I no longer seem to have the original source. However, I doubt this version as the flag could potentially have interfered with Senna’s feet around the pedals.
It is worth remembering it was Joseph Leberer’s birthday on the Saturday and that he was also Austrian. Senna asked him about who Roland was at the dinner, and probably also asked him whether there was something he could do in his memory. I wouldn’t be surprised if Joseph Leberer suggested the flag idea.
To confirm this one hundred percent you would need to go directly to the source – Maria Theresa Fiandri in the Maggiore Hospital or to Ayrton’s ex-manager Julian Jakobi.