In a lot of my work I use Krita in conjunction with Blenders compositor, which allows me to use a single mask to control various layers, control filter parameters through either masks or as derivates of existing layers (e.g. using the lightness to control the intensity of a blur). The downside is that I have to save out any files before using the compositor and I can’t see the effects of changes I make to a particular image until I’ve comitted to them. I’m aware that this would likely require some fundamental changes to Krita (it seems like the objection to the feature request for layers with different dimensions applies here as well), though backwards compatibility could easily be accomplished (since a layer stack is a special case of a node tree). I’m not sure if there are experimental branches for Krita, or where I would start to look at the code if I was going to try to build a prototype.
Hello and welcome to the forum!
My knowledge of Blender is not sufficient to say anything specific here and node-based work is completely foreign to me, but could one of the three plugins connecting Krita with Blender possibly help you?
Each of these plugins has a different purpose and again, I’m not really sure what they are for, but maybe one of them could help you with your project?
Blender Krita Link: plugin for texture editing in krita
Plugin: Blender Layer - Live 3d View in Krita
View3D plugin to view/modify 3D mesh with the current image as texture
But with Layer Nodes they don’t have to do!
Michelist
You would have to rewrite the way the images and layers are represented in code and the whole mechanism that composes the images, not an easy task. Only a few developers are aware of all the details. Maybe only Dmitry right now… The code for that is spread into multiple files. You can start by looking inside the libs/image folder. The image and layer files maybe. I’m not gonna lie, it feels like a monumental task even for someone like Dmitry.
Now, if what you want is only to reuse the layers, taking less memory and the ability to draw in one place and use the results in different places on the layer stack, without copying layers, maybe it would be better to have something like clone layers but also for masks. Its not as versatile as a full blown node system, but it is something way less complex to realize.
The first plugin would make my workflow a lot easier, but it’s pretty unstable on my system. The other two are interesting, but don’t really help with what I’m doing in particular.
Looking through the repository it seems as if the way layers are handled is already as a form of node tree, maybe it’s not completely out of the question. But, yea, it is probably a big request. I think the only painting app trying to implement something like this ATM is BlackInk, but they’ve gone on one of their long update breaks.
Sounds as a “Blender Layer” similar to “File Layer”. But with ability to trat some aspect of the file as a pixel layer in krita 
I would be interested in seeing a feature like this inside of Krita as an alternative to the traditional Photoshop layering system.
My current challenge is that I keep finding myself having to duplicate layers and then deleting/reduplicating those same layers when I have to make changes to something. This becomes very time consuming and tedious over time. That, and due to how I approach certain things (like lights being in a hard light layer with a layer mask), I can’t easily reuse the layer mask to keep things like subsurface scattering in the shaded portion only.