I saw your call for help with regards to custom signals, and I've tried it a bit myself, anyway, here's how I would do it.

1. Make the compression chest itself a constant combinator entity instead
2. On construction create an invisible 1-slot chest entity on the same spot as that constant combinator
3. Set the signals on this fake constant combinator using set_signal (see: https://lua-api.factorio.com/latest/LuaControlBehavior.html#LuaConstantCombinatorControlBehavior)
4. Don't forget to create/update/destroy the invisble chest together with the constant combinator entity

For now in my own game I've added a LuaRemote (https://lua-api.factorio.com/latest/LuaRemote.html) interface to get all the chest info, and use LuaCombinator to export the signals

I will take a look, but I have been pretty busy lately with work (hence this late reply). This level of modification would take a good amount of time and testing that I do not have at the moment.

Thanks for the heads up, I will read up on these two Lua classes in case I find some time to work on it.