You have a good idea.
But the leech list will be more complex if I separate anonymous proxy sites from others by using some symbol.
It will make the users who want to add new sites into the list confuse.
My solution is that you can make a another leech list which only contains the anonymous proxy forums. Rename it to "forumlist.txt" and backup the old one.
Note that not all the proxies in the "anonymous proxy site" are really anonymous and vice versa.
And in most cases, it's difficult to judge whether a proxy site should be included in your "anon list".