I haven't planted broom corn specifically as tomato stakes, but have grown broom corn so I could use the dried seedheads in autumn flower arrangements.

Broom corn has tough, tough, roots. If you plant it in heavy clay soil, you'll never get those roots out of the ground.

My broom corn grew quite quickly to a height of about 10 feet. I planted it at the same time I planted sweet corn, but on the opposite side of the garden.

We are prone to severe thunderstorms, hail and tornadoes. The broom corn is fairly strong, but blew over during a severe thunderstorm with wind in the 50 m.p.h. range. I don't know if I would use it as a living tomato stake? I would be more worried that the broom corn would be competing with the tomato plant for water. But that's just me--our summers are so hot and dry and I have to water the garden too much as it is. Also, FYI the worm that is known as the tomato fruitworm is the same worm known as the corn ear worm. Is planting broom corn near tomatoes going to attract more worms that might bother your tomatoes. Just a thought.