What are the dangers of emulating a battery with a step-up transformer and a 5v power supply?
I would recommend against your approach and use the solution that the camera manufacturers have already made. Every camera brand already has a solution to this assuming you can get AC power to the tripod position.
The solution is a Camera AC Power Adapter, these work by having a dummy battery that is connected to an AC powered regulator. This will ensure you don't cause any damage to the camera as most of these battery packs require the thermistor reading as well as the voltage.
I would not recommend connecting something DIY to the camera, it would be a sure way of voiding a warrenty if you did cause any damage.
Various models for the systems
Sony System (Alpha) - Sony AC-PW20 Replacement AC Power Adapter
Canon System - Andoer ACK-E18 LP-E17
Nikon System - Nikon EH-5 plus EP-5 Replacement AC Power Adapter