Now I have both Senryu and Haiku booting from extended partitions on the EeePC

After much trial and tribulation, I finally got both "versions" of Haiku installed and booting from logical partitions on the EeePC.
Everything is explained in detail in this thread.
The keypoints were:
1. The "makeactive" line which I had insisted in putting into the Grub entry after the "rootnoverify" line was preventing the logical partition from becoming bootable and gave the error

Error 12: Invalid device requested

Nevertheless, it seems to cause no problem on a primary partition.
2. Even after removing the "makeactive" line, trying to boot the logical partitions gave this error:

Error 18: Selected cylinder exceeds the maximum supported by BIOS

My logical partitions started around 1800 cylinders and I had seen where older equipment (which this machine certainly isn't) may require booting below 1024 cylinders.
So, I repartitioned the 16 GB drive so that my extended and (single) logical partitions were at 770-869 cylinders.
However, to my surprise, trying to boot from here gave the same error 18.
So, I re-re-partitioned, this time putting the extended partition at the very start (1-100 cylinders.
And this time it booted.
So, flushed with this success, but mainly because I feared that the logical partition always had to start at cylinder 1, I installed Haiku (the first partition was Senryu) on an adjacent logical partition (cyl 101-157) and this too booted.
Apparently then, while Primary Partition can be booted from anywhere on this 16 GB disk and can tolerate the "makeactive" line in the grub menu, Extended partitions can't and must be booted somewhere below 770 cylinders (probably 512).
You can check the geometry (c/h/s -- cylinders/heads/sectors) of any drive with the command

geometry (hdx)

and you can determine the exact cylinders for every partition using

sudo fdisk -l