Is it possible to define custom io_managers for graph-backed assets? If so how can it be achieved?

yes - if you want to use a custom io manager just for the graph-backed asset, you can change all of the

io_manager_key

fields of the outs in the ops used in the graph to some separate key, and then set that key to your custom io manager.

Is that also possible to do for inputs? For example if I have an upstream asset from which I want to load an input in a custom way. I tried it for inter asset loading and it works but I cant get it to work from a graph backed asset. That would be the case I am interested most in and where I got stuck currently.

on a per-op-basis you can set the input_manager_key field within the In of an input, and that will be respected when constructing the graph-backed asset