Please note that everything I am putting in this reply is my interpretation (putting that in as I do know people on that committee)
I have enboldened the part in your quote that contains your answer
"3.5.4:
MAINS
low-voltage electricity supply system to which the equipment concerned is designed to be connected for the purpose of powering the equipment."
From what you have said your box connects to a solar panel that is supplying your power. This panel is part of the equipment that you are suppling.
The mains as referred to in the standard is an
external supply system for getting power into your unit - generally an AC source. For the purposes of how I understand you are making the connection to the panel would be part of the system you are designing.
If you are in doubt ask yourself a question - if you did touch any of the conductive bits would you get hurt? If they shorted, would they start a fire? Are you liable to get a transient on the connections if it came disconnected while curent flows? If you do, would it contain enough energy to be a short hazard (as defined in section 6, there are couple of nice graphs that show the hazard levels although the hazard levels have been reduced in amendment 1)
From that you should at a minimum be able to construct a technical argument to justify what you are doing.
Also, remember that as far as you know I am just some shmuck on the internet and there are a lot of people out there who will give advice that is not necessarily any good. Fortunatley this forum is generally free of those and when they do appear they get corrected by more knowledgable people. However if you are still concerned then you can seek advice from a notified body or test house.