Afraid I can't help with custom Liquidsoap scripting (it's essentially an entire programming language that you have to learn if you want to use it effectively), but I can at least let you know about the Centova Cast part of it.
If you check out etc/liquidsoap/centovacast.liq (under your stream's directory) you'll find a set of centovacast.callback_xxx values. You can override those functions (put your implementations in etc/liquidsoap/custom.liq) to alter the input sources and add your own.
For anyone familiar with Liquidsoap scripting, that should be all they need to know to tie custom code into Liquidsoap without losing all of Centova Cast's integration features.