... but I would let them know that if something goes wrong there could be a shut down anyway. At least if they do it when you ask then they can control it.
That is an absolutely critical point. If you arc-flash the panel (quite apart from the potential injury and damage of this event) the servers are going to stop dead, and power will be off until some AHJ allows them power be switched on again, assuming there's enough left of the electrical system to support a restart. Servers that lose power at best take longer to come back up than servers that were shut down properly.
In a properly designed data centre there is never any requirement to work hot; its only 'cos they've been cheap on the installation they now find themselves having to schedule an outage. See
Wikipedia article as a reference, in particular the terms "Tier 3" and "Concurrently maintainable".
Make sure they are aware that this is their choice of cost model, there is nothing inevitable about this...