Not quite what you are after, but we put readv and writev into the
library as a step towards putting them into the kernel interface.  The
notion was to permit scatter-gather I/O in user space by connecting
the segments of the I/O request into blocks (Blocks) rather than
copying them several times assembling them into blocks.  This would
also improve the implementation of devmnt.  I'm not sure how much work
has been done in that direction since that first tentative step.

-rob