Can USART TX on the STM32f4 be open-drain?
Although the typical STM32F4 library API defines a "mode" for GPIO which encompasses both the gpio vs. alternate function selection and the push-pull vs open-drain selection, these actually have distinct configuration bits at hardware level, and a define or enum for an open drain alternate function is provided.

However, this is distinct from the question if an open drain alternate function (USART) setting is electrically appropriate to your circuit goals.