Due hardware problems, I'm unable to use any pre-boot software (u-boot, recovery-boot, backupmenu). I'm tired of configuring after reflash and I'd rather create an image to restore. It doesn't matter for me by which tools it's made, as long as they're feasible on a Linux box. What's the best way to do it? I thought about dd-ing rootfs and optfs, but it doesn't sound optimal to me.

As far as i know, there is no way to dd the roots or the kernel, but you could use dd from the PC via backupmenu for optfs, MyDocs -or simply put- complete internal card. Make sure the kernel version between N900, rootfs and optfs is the same, whichever you choose, but fastest might be nokia-kernel.
There is some info about 'building' a flashable rootfs image in the dedicated thread for backupmenu http://talk.maemo.org/showthread.php?t=63975 in the first post.

After initial flash, you would flash the rootfs image, then boot into backupmenu and restore your emmc image with dd.

I have never tried that but 0xFFFF flasher has option to dump images.