I don't know about the safety, but it's not important because this is a poor idea, and quite ungreen. For solar panels to be green, you must use their power. Trickle chargers which spend 99% of their time throwing away their power into already charged batteries are a net energy loss -- it takes a lot of power to make the solar panel at the plant. They only make sense in places that other power is not available, such as a car sitting unused for a month. In a UPS, there is plenty of power.

Or do you have another purpose in mind, ie. doing this on a UPS that is not plugged into the wall, and does not have a load on it, as a way to recharge the battery, very slowly, for short bursts of usage later?