The only place I've ever been where solo diving was officially sanctioned was Bonaire. Other than that most ops are very reasonable to photographers once they have judged that you are not a liability.
Still accidents DO happen. One UK inland site will no longer even let you snorkel solo (without a debate at least) after a well known UK dive industry 'icon' nearly drowned while snorkelling testing out a new housing!
edit - Nope I will not divulge the name!!
Probably more HSE bullsh1t.
Many photographers and rebreather divers are solo divers for all intents and purposes.
It is about the state of mind of the diver and their capability to react to preserve their life rather than panic that dictates the safeness of the situation. If you want to solo dive at 50m then I would say that you were taking a risk but at 10-20 metres then I only ask that you plan for an equipment failure and be aware.