unfortunately due to the goodie brigade you cannot protect your own anymore,, correct about the paint, you can be sued and the same for barbed/razor, though I think in places you now need permission to use wire,, even the burglar ect can sue you now, and the so called justice system will use this as a drop the charge thingy,,
as for plants there was a case a few years ago whilst I was living in the midlands where I was painting the outside of a house for an elderly couple,, the backgarden backed onto a large carpark, one night a couple of yobs climbed over and attempted to force back door and window though they were disturbed and ran towards back wall to get away, don,t know the name of the plant, decent bush with spines about 1-2 inches long, well one of the lads ran into this and got caught up was quite badly scratched ect,, the lad was later arrested by the police, the couple were warned about the bush and were told they could face counter charge?? i think nothing did happen but as said this was a few years back.
personally I think you should be able to use witihn reason whatever you can to protect your own property..
is there a way your commitee or council will supply some of the metal containers, shipping type,, these are readily available and very secure,, maybe one centrally based or smaller ones around the site with the holders having keys to secure tools ect? some i have seen on building sites have had the internals made into individual areas so maybe a small space for each holder?