I guess I've found the answer :


# cd /sata2/BUILD/src/distrib/cdrom/macppc_mkboothfs
# make

This produce the following file : macppc_mkboothfs

# ./macppc_mkboothfs macppcboot.raw

Then run the MKISOFS command as usual, using this new MACPPCBOOT.RAW  
file with the -boot-hfs-file option :

# cd /sata2/BUILD/release.macppc
# mkisofs -o NetBSD-4.99.3_macppc_20061007.iso -hfs -part -map /sata2/ 
BUILD/src/distrib/cdrom/hfsmap.lst -hfs-bless ./macppc -boot-hfs- 
file /sata2/BUILD/src/distrib/cdrom/macppc_mkboothfs/macppcboot.raw - 
l -J -r -L /sata2/BUILD/release.macppc

Seems to produce a valid iso image ...

can somebody tell me if I'm right ?

Best regards