My FLAC collection's on my Synology NAS, and that has ffmpeg installed. So I'm just gonna leave this here:
#!/bin/sh
# Put this in 'flac2mp3', chmod +x
source=$1
oldroot=$2
newroot=$3
scratch=${1%.flac}.mp3
dest=$newroot${scratch#$oldroot}
mkdir -p "$(dirname "$dest")"
ffmpeg -y -i "$source" -vn -acodec libmp3lame -q:a 2 "$dest"
find /volume1/flac -name '*.flac' -exec ./flac2mp3 {} /volume1/flac/ /volume1/music/ {} \;
Note: I've not actually tried it on the NAS, yet, but it seems to work OK on my Linux desktop. I'm going to play with the metadata settings next.