It _was_ a permissions problem
(stupid me - forgot to tell cp to keep file attributes when creating the 512MB data image from the 48MB one...)
I just recreated the 2p9 512MB image and dd-ed it via recovery console.
WORKS
Finally got rid of the Link2D crutch...
BUT: this procedure is not for beginners... I'll better not write a wiki howto
Regarding parted - there's no parted on recovery console
Had to stick with fdisk... but all went fine. You just need to know the size of a cylinder and the size of a sector to compute the correct starts / endings...
Anyways, everyone familiar with dd, fdisk, mounting loop devices, mkfs, etc. should be able to do it:
0.) on the T1, make a backup of /mnt/sdcard to /mnt/extsd/backup
1.) use recovery console to dump all mmcblk2-partitions, mbr, etc. to external sd
2.) use fdisk
in sector mode to alter partition table according to specs above (delete partitions 3 and 4, create extended partition 3, primary 4, logical 5...10 and be sure to place 5...8 at the same starts/ends as before so there will be no need to dd them back...)
3.) power of the device
4.) recreate 512MB empty data partition (dd if=/dev/null of=/tmp/sdb1/mmcblk2p9_512.img bs=512M count=0 seek=1 && mkfs.ext4 -F /tmp/sdb1/mmcblk2p9_512.img)
5.) fill new data partition via loop back device, be sure to keep file attributes
you may include the stuff from sdext2 if you want!
5.) recreate update.img from sd recovery package and replace /home/user/mmcblk2p9.img and mmcblk2p10.img
6.) do a sd recovery to flash it
7.) device should boot now, run fixpermissions.sh and
8.) restore backup of /mnt/sdcard from /mnt/extsd/backup