I'm maybe little off-topic here but is it possible to use the 256MO NAND for something other than a compressed rootfs?
Something like an additionnal swap partition, I don't know much about the N900's initial boot process but maybe keep a small UBIFS partition if NOLO is harcoded to boot on it, and load maemo entirely from EMMC (like some already did with microSD card)

I believe that choosing UBIFS instead of standard ext3 was a really bad decision from Nokia, I think they initially wanted Maemo to fit entirely on the small chip so they used a compressed fs.
With PR1.2 Maemo wouldn't fit anymore on the small chip so they decided to "optify" parts of it but they didn't revert rootfs to something more standard (lack of time/developers I presume)

I will test CSSU-thumb as soon as I recieve my spare N900