Ummm... don't have my tablet right now, but I could easily write something up.

Basically, loop looking at some file in /proc/$(pidof mmcqd)/ for the total processor time used to date, subtract that from the last value, and if it's significant, then it's accessing the mmc. You probably want to do this somewhere other than shell script, if you want a visual indicator, but that would have been my POC. Find some serpent-lover (who has their tablet to hand) to convert that idea to a GUIful Python app...