You might look to something like the silkroad for a case study; the guy(?) who runs that site is pretty active on the forums and would likely give you some time if he agreed with your mission (he has a strong anarcho leaning, I suspect he would, in this case).

Spam can be relatively easily avoided with some captchas; but I don't know how you can avoid fake reports; or how being anonymous even helps or hinders the likliehood that reports would be fake.

I agree with other posters that recommending, teaching, or even requiring them to use Tor is a good idea. tormail for example is reachable from the clearnet, but to actually use their service you are required to connect to their hidden service, which has the added benefit that they are basically impossible to shut down; even if they get your clearnet server (which really just has instructions and metainfo on it); the actual server doing the work is nearly impossible to find, and if it is found but the key is kept safe you can just pop it up again.

It's worth considering protecting not only your clients but your selves; running as a Tor hidden service can go a long way toward this.