Meteor core dev here! We are super excited about Rethink and Slava and I have been talking for a while about an official integration. We scoped multidatabase support out of the upcoming Meteor 1.0 release just to get it out the door faster -- we need to support people using Meteor in production with frozen APIs, and we need to do it yesterday -- but support for Rethink is something I'm very interested in exploring in 2014.
As for the polling on Mongo, you'll love what Meteor is shipping this week. Meteor now by default connects to Mongo as a replication slave and slurps up the replication log to drive your realtime queries.