hi,

...sending a break on TXD

Sure, writes to a ports` bits which are controlled by crossbar, have no effect on the states of these pins.

...debouncing an interrupt by reading the pin inside the handler

It is still possible! Datasheet says that "a read of port bit will always return the logic state present at the pin itself, regardless of whether the crossbar has allocated the pin for peripheral use or not ... with exception of read-modify-write instructions"

Regards,
Oleg