Lately I came up here with a similar question. I want to color special words in all files independet from the active syntax-file. It works with words like DONE and ADD. I tried to achieve the same with [+] or [x] and [-] but it doesn't work. [+] and so on them not to be interpreted as keyword. Fooling around with iskeyword+=[+] and escaping the bracktes [+] didn't help. The following line is in the .vimrc:
syn keyword tododone DONE ADD \[+\] containedin=ALL
As mentioned DONE and ADD work but not [+]. Any help appreciated.