What I tried so far with no luck was to delete swap and recreate it. If I do it from my PC, UBUNTU (gparted or mkswap) always adds a UUID. In fact all cases I looked online were about people complaining about a bug that created partitions without UUID and they were looking for a way to fix it. I tried to cause this "bug" (2010) but it seems is well fixed now.
I also tried to create the swap through backupmenu's root console in case the bug was still there for N900 but I couldn't (empty partition then creating a swap with mkswap, still getting UUID).

Based on my experience, to give your swap a UUID unintentionally happens easily. If you move (like my case) or enlarge the internal swap, especially using gparted, it gets a UUID.

Also, as a result of my tests N900 now only uses internal swap (768MiB) and completely ignores SD's swaps (also flopswap only sees the internal swap). I really don't know how I managed to do this.

Maybe I am missing something since I am not a linux expert, so I would appreciate any help.