How I've seen it work in the past, it'll totally work with GI (and more generally, raytracing). If the frame to be rendered is CPU bound rather than I/O (because of heavy scenes), the whole project would be farmed out to the works, so they have a full copy of what's to be rendered, then resize which part of the frame to be rendered by them. Normally this would happen locally, and if you have 8 CPU cores, each one of them get responsible for a small size of the frame. Now if you're doing distributed rendering, replace CPU core with a full machine, and you have the same principle.
Obviously doesn't work for every frame/scene/project, only if the main time is spent on actual rendering with CPU/GPU. Most of the times when doing distributed rendering, CPU isn't actually the bottleneck, but rather transferring the necessary stuff for the rendering (project/scene data structures that each worker needs).
Why are you thinking GI wouldn’t work? Slicing the image plane pretty much works for parallelizing GI just as well as it does for raster. It does help to use small-ish tiles, that way you get some degree of automatic load balancing.
Funny thing, you sure can! Distributed rendering of single frames been a thing for a long time already.