Ambient Occlusion can be baked with the lightmaps in UE4.
So as long as you have a second UV for Lightmaps set up, it will be fine.
As for adding some irregularity you might want to apply a dirtmap to the material.
Note that you can choose the UV channel you want to use for a texture.
In that Example i baked a Shader in C4D to the Lightmap UV channel (2k texture):
Then applied it as a second TextureSample to the materials using a Multiply node
and used the TexCoord node to set it’s UV to use channel 1 (lightmap channel) instead of channel 0 (diffusemap channel)
Also didn’t use any U and V tiling, because unlike the channel 0 the Lightmap UV channel isn’t layed out for beeing tileable.
In a more advanced state you could do things like using this grayscale image as a mask for a second Texture:
