I'm afraid there's no way to control which addresses are assigned to who. Not unless you put in two separate MXs.
I know that clients connecting to VPN do show up in the clients list and you could assign a group policy to them with their access rights. The problem is that I'm not sure if those entries are the same everytime they connect to the network. The name of their entries seems to be a MAC-address but I'm not sure how it's generated.
If you have a lot of clients this would become problematic anyway because there's a limit to how many clients you can manually assign group policies to: 1000.
Client VPN funcionality on the MX needs some work... I know anyconnect support is planned, definitely get your question in over a "make a wish" too. It makes sense to have such a feature (role-based access for client VPN) and could perhaps be added when they overhaul the client VPN functionality.