Well I think I'm getting pretty close...
In hindsight, not too surprising, but it appears that the format/partitioning of this Olive disk is borrowed from the Mac/PPC Linux, and uses a scheme known as the Apple_Partition_Map. Its pretty well documented, for example, at https://en.wikipedia.org/wiki/Apple_Partition_Map
The partition seems to be in /dev/hda1 and is 31Kb in size. Its in big-endian format.
There is available a MacOS utility pdisk that may be able to edit this partition table, but it also appears that I can just write in the right numbers into /dev/hda1 to resize /dev/hda7. Then as long as the existing tools are enough to re-format /dev/hda7 (ext3 with journaling), it should be good to go... Will probably try it soon.
Also I noticed a picture on the Internet that would seem to show that someone replaced the PATA IDE drive in the Olive with a SSD SATA drive using a general PATA to SATA adapter. I may try this too, to at least use a more modern SATA drive (probably 2.5").
So I think the notion that the Olive uses some proprietary special header on its disk is probably wrong -- its simply that its disk can't be interpreted using an Intel PC since the partition table is not MBR or GPART, but rather Apple_Partition_Map.