Hum - I don't think the Booter supports FFS v2 filesystems, yet. OTOH, sysinst shouldn't create any, at least not for the root file-system.
There are two ways around this:1) Create a small FFSv1 partition towards the beginning of the disk (perhaps right after the Mac OS one) that'll have at least the kernel. You can then have a large (any size) FFSv2 as the rest of the disk and set the root to that.
2) Keep your kernel on your Mac OS volume and boot it from there. Once the system is up and running, install pkgsrc/sysutils/hfsutils and use hcopy to update the kernel from the NetBSD side when it's time to update.
The Booter does need to be fully modernized one of these days. John