That's indeed interesting. It's listed as reserved in the last few versions of the user manual. Earlier versions showed how serial bootload modes was entered and only lacked details on how to use it. I've been asking questions about it and the response I've been getting is that it's not supported.
To quote: "We recommend to use the JTAG for flash programming The serial bootloader from the BOOTFLASH will not be will not be available."
That's pretty hard to misinterpret but maybe there's a language issue.
I'd be delighted to learn I was wrong.
Robert