to get a 5 piece bravo on a standard 10x20 field, none of the pieces may be an S, Z, or a T. This is very unlikely with a history based randomizer, but it is possible.the easiest one to get is O,L,I,J,O, as that has the fewest repeats. You may hold one piece in TI, which will increase your chances. It is impossible to open with a 5 piece bravo with bag, as you are guaranteed of getting at least one of S, Z, or T, and if you hold it you will be getting another of them.
It may be unlikely, but it's certainly possible! I was testing muf's mame.ini settings when I was surprised with a bravo double earlier today. It happened in TGM 20G mode at the start of the game. Time was 00:03:xx and level 6. I was about to take a screenshot, but for some reason I quit the game instead. And I didn't record it... Maybe next time. ;