implementation specific registers in elf core files?

I would like to save implementation specific registers in elf core files,
and have gdb find and print those registers under the guidance of the
target description machinery (xml reg specs) described in Appendix G.  Is
this possible? I would presume the values would end up in an ELF core file
note, but what would the note's name and type be, and how does it all come
together?

I suspect gdb has most of this already, since the gdb server can send
upstream the xml data, and also the value of the registers, and it somehow
comes together.  (I have no practical experience with this.)