You'll get away with it probably but they will know you did it. Since you say it is a Skymiles ticket (I take that to mean it is an award ticket and not a purchased one), you will get penalized or, possibly, booted out of the program. I agree that the $150 fee seems really stupid. In fact, it's incredibly stupid. It would be one thing if you had deliberately purchased the ticket intending to dump the final segment but it's a non revenue ticket to begin with. (OK, it's dumb in any case to make you fly the final segment of a trip if you don't want to, regardless of what kind of ticket you bought or how much you paid for it.)

This is one of those things that makes dealing with the airlines so maddening sometimes. I would tell the agent at check-in that I was not going to fly the final segment and ask them to only check the bags to the connection city. A thinking person standing at the counter can do that, I think, realizing that it makes perfect sense. If they won't do it, then proceed with your plan. You haven't given anything away. They are going to know that you didn't board the final segment anyway.