I am sure there is something you can put in asound.conf for this, some googling on alsa and what youre after should find results. however, that file is managed by lmce as youve found out and is overwritten at boot time. you would have to dig into the source and modify whatever it is that writes the config file.
a better solution might be to use a dedicated md for your distributed audio system, maybe even a squeezebox would work.