As of version v9.5, you cannot specifiy any credentials for Pre-/Post Scripts in Jobs. Nicer would be to be able to provide Credentials, much like you already can specify (from the "managed" Credentials). This would allow you to write and use scripts that can do network-access. Right now, by default, VBR runs under the NT AUTHORITY\SYSTEM context, which does not have network-access, so any attempts to use it, for example, a simple Get-VM -ComputerName <Server>) would fail. Awkward part is that Veeam will state at Job-Run that your script was exectuted successfully, which in fact, is the reality, since Veeam does not know any networking-related commands inside your script actually failed.
The Pre-/Post are like "traditional" Pre-/Post in the sense that you cannot schedule them in any way, not inside the Job-time or outside of it. Perhaps it would be nice to have a new Job-Type, for example a Script Job, which would give you more flexibility to alter/prepare your environment for whatever you need before, during or after backups/Replications. I do realize Windows has the Task Scheduler for this, but having everything backup-related in one place together would be nicer (and as such could also be "replicated"/Copied using a Configuration Backup File Copy Job, for exmaple to your offsite VBR-location).