Seems like the NVIDIA Ansel plugin has been deprecated, at least it’s no longer part of the engine. I couldn’t find its source code either, but instead I stumbled upon a different image capturing plugin on github with an in-depth tutorial on how it was made.
Its readme explains in detail how you can request rendered images from the rendering thread. This alone is not enough to solve my problem, however.
For now I’ve settled on a different approach: I use a scene capture cube with Capture Every Frame set to 1 and Capture Rotation set to 0 to render the cubme map every frame, then use a regular camera with a post processing material which, using the formulas I compiled, draws a fisheye image from the cube map over whatever the camera would normally see.
Maybe one day I’ll find a better solution. But still, thanks for your support so far @Roy_Wierer.Seda145.