Jannis,
I am experimenting a bit with the guts of this.
If I have rewired my jack graph correctly, I have liquidsoap taking in the soundcard audio and sending it out as normal, at specified times, the same liquidsoap instance plays the files and sends it to the jack graph as alert.
I have normal audio going into the main inputs of the compressor, I have alert audio going into the sidechain inputs of the compressor. I have the compressor outputs and the alert audio going out to the soundcard.
It seems to be working as well as what I was doing before and seems much more simple...
More experimenting to come.
Thanks for the help everyone.
all the best,
drew