I have a cynical view on people in general and can see the anonymous account being easily abused.
The PM the mod option is good because it self selects on multiple levels.
First, the original user must be willing to poke a mod to do this. Second, the mod will decide if it is justified. At this point they can post, or if an anonymous account is needed, have one that only mods can access.
This would provide a bit more anonymity since users wouldn't know which mod did the posting.