Mounting a server by only its ears?

Only if you don't have any problems replacing it after it falls. I wouldn't chance it.

IMHO, No. Servers are meant to be installed on rails or shelves. The "ears" are meant to secure the server in the rack, not to support it.

Look at the server's manual. If you haven't bought it yet, look online -- there's normally a mounting guide of some sort, and it might have specific instruction for dealing with a two post rack.

Some systems have alternate holes tapped in the sides so you can mount the ears closer to the center of gravity. The system will stick out the front of the rack, but it'll cause less torque.

Other systems have enough of the heavy components far enough forward that they're fine with 2 post mounting with only sticking out a few inches. (basically, you turn the ears around before attaching them).

Another option is to get a shelf rated for the weight, and install that to take the load of the server.

I'm sure it is possible, but if it isn't designed to be mounted that way, then I wouldn't risk it.

Is it possible? Yes

Is it advisable? No