I'm working on architectural visualisations, and I can't imagine more useful feature. How render farms for rent do that?
I'm not sure if a render farm currently exists to do this, but theoretically it's easy. Simply use a method called Image Stacking, where you render the same image multiple times with different seeds and then mix them together to average out the noise.
The first two images would mix 50/50, the third would mix into that by 33.3%, the fourth 25%, 5th 20%... basically 1/n where n is the number of images in the mix.
This is the method that renderfarm.fi uses, but I believe they have a minimum animation length so you can't use it to render a single still image.
A more makeshift solution might be setting up cameras to render small sections of the scene, and then stitching the, together with a panorama program. Could take a while, though.
Actually Blender is very well capable of slicing a single image into rectangular tiles and dispatching them to different workers. It just has to be scripted.
A distributed render engine that does this is BitWrk (https://bitwrk.net/). It has tiled rendering for stills and animation, conveniently integrated into Blender. No more zip file uploading. You can build a private render farm with BitWrk, or use its peer-to-peer service, or mix both.
Disclosure: I am the person behind BitWrk, so feel free to ask me anything about it.
You can send a single image just make sure that if you don't have a single frame render option just make sure you've set up the project to only play 1 frame.
You can all so set up a local system to export it from cycles which can do single frame renders.