Ok first of all, i have no solution for sidecar but:
I installed the new update 10.15.4 and before that update my ryzentosh was not able to even regocnoze my ipad pro on the sidecar options. Now it connects and my ipad screen turns black. It seems to work somehow and even the ipad opens up the sidecar app. But still the screen is black.
So i know that it requires an iGpu but maybe some smart guys out there can do something with that information and we come closer to a working sidecar on ryzentosh opencore!
I hope i can start a discussion and if any information is needed I'm pleased to help you!