The previous feature therefore allows you to remesh different parts of the model independantly, like I did with this model of a Napoleon statue on Sketchfab, remeshing details such as the sword separately from the horse’s body:
To do so, you’ll have to separate your model into different “submodels”, while keeping the boundaries between them intact:
You should now have as many objects as regions you created, which you can then remesh independently with the MMGS button, specifying a custom hausdorff ratio for each of them (keep in mind that the ratio is with the selected object’s size, you’ll therefore have to adjust!).
Once all your remeshed parts have been re-imported, you can then join them into an unique object by using Join or Ctrl + J, and jump to Edit-mode to Remove doubles vertices.