I realise that this is ancient history, but I am trying to piece together the purpose and current use (if any) of the 
fpscr_val field in PPC_FPU_REGISTERS.  Does anyone recall why it was introduced and what it was/is used for?

As far as I can tell, it isn't used any more except by gdb, which uses it (apparently incorrectly) for the value of the 
$fpscr register.  It looks to me almost as if it was introduced into the kernel with some sort of master plan in mind, 
but nothing ever ended up using it.