Keystone issues with Meteor

I'm trying to use Keystone as an admin back-end for a database populated by a Meteor app, and I'm running into a few issues.

1. Only a single item appears in a list.
2. The item's URL is keystone/posts/null.

@michaelrambeau, who also ran into the same issue when trying to use Keystone with a different Meteor app, suggested it might be due to the fact that Meteor uses strings as IDs instead of MongoDB object IDs (see idGeneration here).

Any idea what I can do about this? I've seen it suggested to add _id: String, to my model, but that leads to a Path _id on list Post is a reserved path error.