Taking the address of the object we want to watch:
Note: I dragged the "Value" of &a as a new watch, to be able to copy paste it.
Adding the new breakpoint that watches the variable's address (WARNING: This can slow things dramatically...)
Notification when the memory is modified:
I didn't get it working on complex structures, but you can most likely find out which member you want to check, and test for its value change.
If anyone knows how to structures, drop me a mail.