This has been on our To-Do list for a while. Looking at the Location Map in your account the plan we have for it wouldn't really help much. Your locations are all spread out all over the place (splitting it on the river looks cool though).
The trimming we have planned would be more to give a common trim boundary to multiple layers at the exact same location like in this sample:
https://www.mapsmadeeasy.com/locations/public/6e2b9d0bdd51419387f1ae5312a2a2da
So you are saying you want the custom trimming for each layer?