I've reflashed the N900 kernel from onboard the N900 using this on-board flasher. That's easily doable, and instructions are lying around. I've never tried to flash the rootfs and mmc with it... I suspect doing those things might just put your device into a bootloop, but if you manage to keep everything needed to finish the flash in-RAM, and don't write ANYTHING to your partitions in the meantime, you may be able to pull it off.

As for implementing it from withing BackupMenu, I suggested it to Robbiethe1st, but I am not sure if he's had time to look at it. You can also mount rootfs and/or mmc from within BackupMenu's command line / terminal - so there's no reason i can imagine why you wouldn't be able to reflash completely from within BackupMenu, in spite or what MohammadAG said about NOLO (which I admit I don't fully understand, which means I may be missing something important).

I do wish that Nokia made fiasco-flasher freely distributable - it's a bit easier to use than the softupd + flasher combo, and it could thus be more easily placed in BackupMenu - it's less versatile, but by the same token it's also more likely to fit the kind of recovery you'd do from BackupMenu - reflash the kernel partition or the fiasco image - pretty hard to make N900 unbootable from the MMC alone, so little need to reflash that from on-board in most cases.

- Edit -

Remembered my reason for posting this: When we (me and some other folk over on one of the uboot threads) were fiddling with flasher for a while, we noticed it didn't work for enabling R&D with the --local flag. So you can't really do that, unless someone figured out how to do it since then.