Mongoose store either ObjectId reference or string

Is there a way to specify a heterogeneous array as a schema property where it can contain both ObjectIds and strings? I'd like to have something like the following:

var GameSchema = new mongoose.schema({
    players: {
        type: [<UserModel reference|IP address/socket ID/what have you>]
    }

Is the only option a Mixed type that I manage myself? I've run across discriminators, which look somewhat promising, but it looks like it only works for subdocuments and not references to other schemas. Of course, I could just have a UserModel reference and create a UserModel that just stores the IP address or whatever I'm using to identify them, but that seems like it could quickly get hugely out of control in terms of space (having a model for every IP I come across sounds bad).

via Turner Hayes