How can this be handled in a community that doesn't have that that extra level above moderators?
It is difficult to imagine something like that. There is always a superstructure that gives the power to the moderators in the first place. That superstructure is the one that ultimately should deal with problems of the Moderators themself.
To put a more practical example: How do you moderate the executive government? Easy, you create another power that regulates them. Both powers in balance. In the example you provide, this is not possible, unless the user base is large enough to provide the means to build the community itself, as a nation could do. In those cases the people are the ones that shall deal with problems of moderators. This normally ends in creating a special forum/commission/group to evaluate and have the final word on the people with power.
As you can see, the one that gives power is always above moderators, the community itself can remove them if they deem necessary via the same method that put them in place.
