No reboot anymore, indeed!
I've changed koreader_kobo.sh to:
#!/bin/sh
export LC_ALL="en_US.UTF-8"
# we're always starting from our working directory
cd /mnt/onboard/.kobo/koreader/
# export trained OCR data directory
export TESSDATA_PREFIX="data"
# export dict directory
export STARDICT_DATA_DIR="data/dict"
# stop Nickel
killall -STOP nickel
# finally call reader
./reader.lua /mnt/sd/.Library 1> koreader.log.`date +%s` 2>&1
# continue with nickel
killall -CONT nickel
And it works.
(If I get problems with the sleepfunction, I'll post it.)