well theres a manual way you can do it that only works in reaper. Use the JS splitter/joiner plugins and set up a normal sidechain-able compressor on the band you want sidechained. Side chain like normal and thats it. It kinda makes a mess of your channel strip and takes much longer but it definitely works.
Edit: I made the same search a few months ago and wasted a good hour getting no results. I think i remember coming across another multiband comp that did it but was also one to pay for. My memory might just be making that up though.