Hi, I'm wondering if it is possible to have swatches and brushes which can paint both diffuse, roughness and metallic channels at once.

Hi, that's unfortunately out of scope of the tool, but I'll add it to be planned for v2.0.0, thanks for the suggestion.

You can follow it here: https://github.com/orgs/PardallTools/projects/2

Thanks for considering this feature. This is what would make me switch from Blockbench to this addon.
Another addition on that is maybe it should be more generalized to not only PBR but any arbitrary channel/texture which can be input for Blender material nodes. So that it can be later wired to anything. The idea is to have several configurable inputs on a brush stroke.

Can you explain what would be the PBR workflow expected, please? Since this is pixel art oriented, I'm trying to picture what would it be like (never used Blockbench).

Sure, the best workflow I've discovered so far is Union Bytes's Painter  Stamp tool (illustrated in their video). It works on multiple channels and allows kitbashing details on already painted models. Technically this stamp tool of width 1x1 is a pen which draws on multiple textures with predefined color for that texture. Blockbench's PBR workflow already exists but IMO it is not advanced and convenient to use (here's the video) because it is not designed paint on multiple channels. I.e. you have to pick grayscale for height manually, green for  roughness, red for metalness etc.