I have travelled as a courier half a dozen times. I have always been able to keep my miles and I have always known in advance which airline I will be flying on. I have not done courier flights lately, because I usually want to travel to SE Asia and right now the courier companies are using Japan Air Lines. When I last flew I was flying United.

You can choose your return dates, usually you will have a flexible return between 7 and 30 days after departure. However, you must choose your return date in advance of your departure and you may not change it without possibly losing your deposit.

Many of the courier flights will allow for one piece of checked baggage, however on a 747 I never have any problem with using just overhead space.

So yes they will work for mileage runs, however, you will probably need to spend at least 7 days at your destination.

There are good organizations out there that you can join that act as a clearing house for courier companies. If anyone is interested in a recommendation please email me privately.