In the meantime I am trying to decide which would be the best fs for the overlay.

Jffs2 seems a good choice in this case, and it also seems to offer some control for compression settings (perhaps no compression is best as N900 doesn't have much space limitations, unless it noticeable reduces write times) and also for setting when garbage collection may be done. I like that it allows for atomic operations and that it is flash friendly.

Garbage collection might be posponed until on charger AND idle, providing the overlay is adequately oversized.

Have anyone done any benchmarks of jffs2 or any other flash friendly fs in N900?

My only intention is to have an overlay for /, with no changes to the rest of the system. The idea is just to have an oversized rootfs just in case **** happens, and also for having multiple overlays for different purposes. All the rest (/opt etc) being unchanged.

For /opt lack of space I am just using a combination of additional loop mounted images and symlinking which works ok for me... I don't see any issue there.