There are workarounds - some other plugin like Bidule for instance hosting Reaktor, you could stick it on the track you want to sidechain in and wire it up in Bidule.
Bit of a fiddle though.
Just a question - I wonder, is the reason that Renoise doesn’t already have this due to it being based on an old audio engine, namelyNoiseTrekker, and the limitations of that, or has the engine been rewritten these days?