No. You can't easily extract firmware image directly to SD card from the tablet. It is possible but quite complicated so I won't go to details. It is easier to do it on linux desktop PC but still it is complicated. You can unpack firmware to pieces and mount rootfs.jffs2 on PC and archive it via tar or copy to card. AFAIK there is no way to extract jffs2 image directly (i.e. no mtd modules in kernel and no mount). The easiest way is to flash new firmware to internal flash, then copy it to mmc and then possibly restore previous backup of internal flash.

Also you can boot both 2007 and 2008 rootfs from card/internal flash but you will not have full functionality. There is only one shared kernel and initfs partition and 2007 vs 2008 differ too much. You can boot 2007 with 2008's kernel and initfs but wifi won't work in 2007.

I am currently playing with kexec idea (i.e. booting more kernels) but it is far from usable state. And this route will be more complicated since you would need to have more initfs partitions too. Perhaps it is too complicated for average person to be practical.