Golem appears to be in early stages of development. The use of blender and luxrender is a deliberate move to get people using the system during the early development. More advanced uses can be expected as the system is developed.
From the FAQ page -
Brass Golem is where we are at the moment with our MVP, in alpha testing now. This current version of Golem is only focused on rendering in Blender and LuxRender ... we consider CGI rendering to be one use case among many, and also a training ground.
From the developers point of view, rendering animations consumes large cpu resources for extended periods of time and only involves passing one command to blender. A single blend file uploaded can be used to keep hundreds of workers busy. This is a good way to stress test your distribution system while you are developing it and you start building a user base in the process.
If you can't wait until development progresses, you will find tasks are processed using docker images, so far only a base system (debian), blender and luxrender docker images have been created. Some limited info can be found here that indicates you could build your own docker image that can perform other tasks. This wouldn't be supported within the main network, so you would also have to look at running your own worker nodes, tracker and modified client.
As another idea, you might be able to abuse the blender nodes. Blender includes a python interpreter that allows users to automate tasks, as well as extend blender through addons. While you may have trouble using more than the base python install, you can save scripts (and text data files as well as images) inside a blend file which you might be able to get running from a rendering task. This may involve an initial script registering a bpy.app.handler that runs at render_init which could perform non-blender related tasks.
