Bem, tive que fazer um script para fazer isso, eh, bom o suficiente:
cat > slowpipe << 'EOF'
#!/usr/bin/env bash
set -eu
data=$(< /dev/stdin)
hz=$(bc -l <<< "1.0 / $1")
while [ -n "$data" ]; do
printf '%s' "${data:0:1}"
data=${data:1}
sleep $hz
done
# XXX It seems to be imposible to save leading newlines in variables,
# you cant have var=$'\n'$'\n' no mater what you do, only one new line can will
# be saved, IFS won't help, or anything. one way is to use 'read', without
# saving all stdin to variable.
echo
EOF
chmod 755 slowpipe
./slowpipe 42 < rpi.cmd >> /dev/ttyUSB0