The text in the reboot dialog cannot be edited. You could use the Remote Dialog option in the General Tab of the Deployment Template to inform the end-users what is going on. It may be a good idea create a feature request: Feature Request
Correct that you can edit this, and we are also doing this.
But where does the Safe Reboot pop-up get the credentials from - is it the user that are logged in, and performing the operation?
Everything is run as the System Account.