Not really a direct answer, so apologies, but I thought it might still help someone.
Lightact media server (https://lightact-systems.com/media-server-for-unreal-engine/) has several openCV functionalities and it works with UE4, as shown in this tutorial:
The video shows how to use FindCircles, but there is a FindContours node as well.
DISCLAIMER: I’m the founder of Lightact.