The side chain in a compressor is the signal that actually controls the gain reduction. It's either spit off before the gain element (feed forward) or fed back from the output of the gain element (feed back). Pipeline XT simply routes the audio to hardware outputs and receives it from inputs again.
If you want to feed a hardware compressor side chain, you can use Pipeline to feed a separate hw output that goes to the compressor side chain input. You can add a HP filter to that signal before you push it out Pipeline as well. no need for any new features there.
Compression side chain filtering does not pass audio through unaffected. Rather the side chain signal has a HP filter processing it so the low frequencies don't trigger the compressor, which is not the same as passing the low end through unaffected.
It also sounds like you might be asking for some kind of multi band split/sum before/after the plugin, which you can already do as well.
