While I can't argue that it *should* be able to be made to work, grub is
a less-used, non-default implementation of a bootloader. As usual,
patches are welcome ...
Explain your need for grub. Or will syslinux do just as well?
Supporting pre-validation of parameters is something we've had prior
development discussions about, but unlikely to have anything done about
it in time for the Jessie release.
Ben