I’ve previously noted a loophole that lets you transfer American Express Membership Rewards points to United Mileageplus miles, even though United does not participate in Membership Rewards. The gist was transferring Membership Rewards points into Continental miles, transferring the Continental miles into Amtrak points, and then transferring the Amtrak points into United miles. (This method also allowed you to move miles out of Continental and into United.) The loophole has been tightened somewhat:

Amtrak limits the number of Points that a Member can redeem for Program Partners or Merchants frequent traveler points and/or miles in a calendar year (including but not limited to United MileagePlus, Continental OnePass, Midwest Express Frequent Flyer and Hilton HHonors). The yearly limit for Amtrak Guest Rewards Select Members is 50,000 Points. The yearly limit for standard Amtrak Guest Rewards Members is 25,000 Points.