For “view and control” of Things across Locations or even across Accounts, on one or more dashboard Panels, you’ll find ActionTiles to be helpful, as we allow mix & match of Things from any Location (Hub) very easily. ActionTiles consolidates your Things in our Cloud.
Unfortunately, for automations, as mentioned, SmartThings currently limits a single SmartApp instance to access Things only from a single Location. I’d bet CoRE can (or could be enhanced) to get around this limitation by implementing http REST-API endpoints for multiple SmartApp instances to talk directly to each other. I’m rather confident it can do this already.