I think the “easier” way is to generate your dirty maps with “render to texture” in 3DS… You can apply a black and white vray dirty map in diffuse and render it to texture using the unwrap channel you’ve generated for lightmaps. Inside UE4 you’ll can create a base material with a “Param2D” node for those dirty maps. Then you create instances for every different mesh and apply the proper dirty map for each one. It’s not a lot of work if you won’t apply dirty materials all over the place…
I think maybe a real time version of dirty generation could be implemented with a variation of ambient occlusion algorithms… 