I think there's some magic in gdb for this but I'm not sure how to
make it happen. If you figure it out I would think it would be
generally useful and we should find a way to put it in the source tree
so it works for everyone.
You might find it useful to put breakpoints in heap_deformtuple() with
conditions that catch the tuple you're looking for. That function will
often (but not always) be the first function to see your corrupt
tuple.