Yes, it is possible to use a SD card instead of the eMMC. There was a thread about someone who did it when their eMMC failed completely, but I can't seem to find it. If you look at the wiki in the Repartitioning the flash section, one of the options involves moving the /home (which also contains the /opt) to a SD card on a temporary basis while you repartition, I don't see why you can't adapt some of those instructions for a permanent move to a SD card.