Sorry, haven’t had much time to hunt this down yet. Could you check if LocalMerge (i.e., landscape.EditLayersLocalMerge.Enabled=1) works? From what I understand, it should be more or less the same benefits, except BatchMerge is backwards compatible with all landscapes, including those using Landscape BP brushes (using Landscape BP brushes still results in similar behavior as global merge, though).
-Ryan