I don't think you can
The pre-freeze-script.bat system during VMware backups is actually a function of how a VMware snapshot request interacts with the VMware tools installed inside the VM itself, and as such it is function Backup Exec uses, but not a function Backup Exec provides. To achieve the same thing in Hyper-V, Microsoft would have to provide a link via their own virtual machine support tools and backup API.
Closest thing I can suggest is use the Task scheduler inside the VM to set something to kick off before the backup starts and then have another task resetting those changes that is timed to run a reasonable time after you know the backup should have finishes. A possibe other option (which would involved some form of remote scripting) is to set pre- and post commands to run on the media server itself and have these commands do some form of remote execution against the VMs. Note you cannot set pre- and post- to run on the VMs themselves as that only works for standard remote agnet backup requests and not virtual agents
