Full reindex took a little more than 48h (I think about 96h), but now it's done and bitcoind works fine on my BeagleBone Black right now.
I will try to copy over this blockchain to RPi and see if it will work there. I had same problem with it before I bought BBB, but since RPi has twice slower CPU, indexing in there would take ages. Yet, maybe with readymade index it will work now.

IO is still a concern, I'm not sure how lond SD card would live with bitcoind running constantly on it. But I will find out, eventually.