Would it possible to export vertex colors when exporting an OBJ file ? 

Threejs supports this format - where each vertex has a color value attached. It would enable users of threejs to efficiently  mesh-merge many objects without having to deal with textures. 

The format is described in https://github.com/mrdoob/three.js/issues/11422

( I am a dev - I can help out if needed ) 

wrote a small typescript method that will do this - It will take an asset forge generated obj and mtl file, and generate a single obj file with vertex colors. 
Obj file will potentially grow, as vertexes that are shared between different coloured faces will need to be duplicated.

I will publish the script if there is a request for it

Yes, please that would be amazing!