This family of devices seem to partition/mount the flash card on the fly, depending on kernel parameters passed at boot (look at the output of dmesg | grep initrd).
If you want to repartition the flash memory to expand the /data partition, you can use RK29xxImageTools to unpack the official .img, replace the kernel parameter file and repack the .img.
Attached you can find the parameter file I use to expand /data to 2GB.
BACKUP THE TABLET before trying because /data and /mnt/sdcard will be erased!!!
The idea is that you can expand /data by reducing the space available for /mnt/sdcard.