A dynamic email isn't going to be possible as you have already found out, not directly through options in MobiControl.

You could:

1. Send a message to the tablet directly using a message box like described here but on the flip side this can easily be ignored by the user by simply pressing the back button or through interactions with device notifications, or just by dismissing the message and not reading it. What we have done many times in the past, though this only works for kiosked devices, is to combine this with a little notification card on the kiosk towards the top of the screen (below the logo for example), that displays important messages. This can't be dismissed (unless you allow it to be) and its a great way of communicating with users.
2. Use the webhook mechanism in signal policies. Trigger a webhook to and endpoint (powerautomate for example), then that in combination perhaps with an asset register that links device to user (because you can't pass user details associated with the device in the webhook) and send an email to the user.

SOTI's new snap based kiosk builder (v2026 of mobicontrol) does actually have an inbuilt mechanism for this where you can switch the device to a differetn lockdown screen if the battery is low. This solution does have an additional cost associated with it though but certainly worth looking into.