That would be a huge loop if you and the scope are in the car. If you can run the probe cable back, surely you can attach a ground somewhere back there? Also, if you are going to be checking the power or other signals at the ECU with the scope as well, don't do both at the same time, or at least only connect one ground at a time. So if you wanted to check the ECU B+ terminal for example, you'd disconnect at least the ground part of your alternator probe and connect the ECU probe to a local ground. I think the alternator check can pretty much be a one-time pass/fail--if no problems are found just disconnect and retrieve your probe and move on to other things. Alternator noise issues are relatively rare and tend to be fairly severe under load--and often audibly noisy as well.