kernel panic when booting with xen-pciback.hide=

Hello,
I'm running alpine-xen-3.0.4-x86_64 and want to pci-passthrough a SATA controller. My system supports VT-d.
The problem is to get the pci device assigned to xen-pciback at boot time.

It all works fine if I boot Dom0 with these lines in syslinux.cfg:

 
KERNEL /boot/syslinux/mboot.c32
APPEND /boot/xen.gz iommu=verbose --- /boot/grsec alpine_dev=UUID=53EB-33F3:vfat modules=loop,squashfs,sd-mod,usb-storage modloop=/boot/grsec.modloop.squashfs nomodeset --- /boot/grsec.gz

and then do  xl pci-assignable-add 00:1f.2 from command line prior booting the domU that has  pci = ['00:1f.2'] in it's run configuration to get the device passed through. The domU gets the SATA controller and sees the attached drives.

But if I add  xen-pciback.hide=(00:1f.2) as parameter in syslinux.cfg, i.e. booting with

APPEND /boot/xen.gz iommu=verbose --- /boot/grsec alpine_dev=UUID=53EB-33F3:vfat modules=loop,squashfs,sd-mod,usb-storage modloop=/boot/grsec.modloop.squashfs nomodeset xen-pciback.hide=(00:1f.2) --- /boot/grsec.gz

the kernel panics with message  /init: /eval: line 1: syntax error unexpected "("
I've tried to remove the parenthesis (xen-pciback.hide=00:1f.2) or use quotes (xen-pciback.hide='(00:1f.2)'). In these cases, the kernel boots up fine, but without any effect, the pci device uses the ahci driver rather than pciback.

Any ideas? How does assigning a pci device to pciback at boot time work with Alpine Linux?
Many thanks!