If your input is 480V, unless there is a transformer inside, the DC bus will be around 650VDC. The DC bus is always at the PEAK voltage of the system,so when you say "480VAC" or "120VAC" that is the RMS (root mean squared) voltage, a form of averaging over time. The peak voltage is actually around 141% higher, and that is the DC bus voltage (for the most part) of anything rectified from AC to DC, such as in a UPS. What you do with the DC to make it back into AC is the magic part of it, but regardless of the value you end up with, the DC bus is always the AC x 1.41 (or divided by .707).
NFPA70E is not a "law", it is a system of guidelines for establishing a safe electrical workplace, which IS mandated by Federal law and enforced by OSHA. What OSHA actually says is that YOU WILL have an Electrical Safety Program, you will review it periodically, and your employees will be qualified on it and periodically re-qualified on it. OSHA is light on specifics, but highly suggests that your program should look some thing like what is described in NFPA70E. Looked at another way, and in the light of how OSHA works, IF there is an electrical accident involving employee injury or death and there is an investigation by OSHA, the first question will be "What is your electrical safety program?". If your answer is "What?" or "huh?" or anything like that, it gets really ugly really fast, including, but not limited to, the filing of CRIMINAL charges for all managers and supervisors involved if they find gross negligence. If on the other hand your answer is "We follow a program that is based on NFPA70E", then the rest of the conversation is going to be about details and execution.
So does NFPA70E specifically address working on UPS systems? No, that's not what it is about. It is about working SAFELY on live equipment, ANY live equipment. Part of that practice of working safely is to establish what that means for any given task. SOME common tasks are laid out for you in the NFPA70E documents to help streamline the process, but just because it is not specifically mentioned does nothing toward relieving you of responsibility for creating a safe workplace. Your responsibility as an employer includes the process of IDENTIFYING the risks associated with a task, and there are very specific processes for identifying those risks as it pertains to Arc Flash energy (which by the way is not the ONLY risk). Some of those pre-defined tasks have tables (as mentioned earlier) that assign risks to them. If your tasks do not fit those, you must do a Hazard Risk Assessment to determine them and the procedures and PPE associated with them.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington