I don't think it's stupid at all, in fact was toying with an idea like that.

However, it's not practical to include the SD card in the LVM, because:

you might wanna take it out and read on a PC, copy data to and fro...

what if you have to reflash etc., then essentially everything is lost, you have no safe haven.

In my mind the SD card should serve as a permanent storage for data, in case anything goes wrong with the main unit, you still have the stuff that's on the card...