The idea is to put a decent load on the engine, hopefully to duplicate the heaviest load it will see while riding. Placing a higher load on the engine will increase cylinder and piston temperatures, which puts different demands on your engine than riding at a similar throttle position with out an additional load. Naturally not everyone is going to have the idea area to duplicate the procedure as it’s outlined. My advice would be to think about the area you ride in and determine what area you'll be placing the heaviest load on your engine and decide if its an area you'll safely be able to apply that load for a decent amount of time. Certainly something like loose sand would be a good substitute or perhaps nice thick mud :)
Running the test in a higher gear is again to place an additional load on the engine. Perhaps you have an area you can run at a high throttle setting but not at a 4th or 5th pace. Then give a lower gear a try. I think likely the most important thing to remember is that you are fine tuning the jetting to meet the conditions you ride in and your riding style. If there just isn't an area you can place an additional load on your engine to do a plug chop then likely its not something you'll ever encounter.
I hope that helps.
David