Problem with mounting to a tree, aside from the method - none of which are entirely ideal anyway, is that if anything ever were to go wrong, it won't put you in the best light... Anyone assessing your work is likely to have their own opinions as to whether or not a tree qualifies as a reasonable place to mount anything electrical. One could argue that a tree which moves/grows cannot be considered a solid/sound support to mount to. Also, trees are known to fail. They can fall over in high wind. Unlikely but possible, and if the first person to go and see the extent of the damage also gets electrocuted they're going to be doubly cross!
Granted the above is quite extreme and I'm sure you can do it safely, but I would be tempted to just say no. Apart from the technicalities above, there are divided views on how best to mount to the tree itself, and a light mounted to a tree also looks a bit weird/diy imo. A simple light post would be my preference. Speaking of which, I was in B&Q the other day and saw some pretty awesome LED light posts, easily bright enough to compare to a flood light.
Granted the above is quite extreme and I'm sure you can do it safely, but I would be tempted to just say no. Apart from the technicalities above, there are divided views on how best to mount to the tree itself, and a light mounted to a tree also looks a bit weird/diy imo. A simple light post would be my preference. Speaking of which, I was in B&Q the other day and saw some pretty awesome LED light posts, easily bright enough to compare to a flood light.