Hey 4.33,
I’ve just been investigating using RYTM sidechain inserts in Studio One 3.5.1 on a Macbook Pro running 10.9.5.
You are correct that the inserts don’t show up in the plugin.
However I have found it is still possible to send to the RYTM Sidechain inserts.
In Apple’s Audio Midi Settings you can set up a Aggregate Device between your soundcard and the RYTM Core Audio.
This means within Studio One you can access the RYTM In & Outs, then activate & label them in Song Setup (Preferences)
From there, as long you have activated the appropriate output channel to Analog RYTM using Overbridge, then you can rout signal to the RYTM and process using the Analog Filter, Overdrive, & the Master FX.
I found there are some glitches at low latency settings, and at high latency there are timing issues, but there may be a way of addressing these.
In the meantime you can still process and automate settings for a drum bus for example, and then mixdown in Realtime in Studio One.
Hopefully you are in Mac and this works for you too.