I do something similar on a very long stretch of 30 mph. residential road. Smokey constantly sits on a 2-wheeler between cars, pointing his radar gun at everyone. I usually set it, then back off to 25 and relax. And wave at the nice Police Officer when I pass. This has saved me from dozens of cites.
For anything 20 mph or less, I just select gear 4 or 5 and let it idle along.
In the Kenworth, I have it set so that it can be engaged at any speed or gear. This is VERY useful when pulling a long (5-10 mile) 12-14% grade with a load on. I set it to 1800 rpm, with the pyrometer is running 900-950 degrees. This lets me keep a close eye on engine coolant temp, transmission temp, rear differential temp and seat covers in the lane next to me. And I can make tiny adjustments if anything heats up or the grade changes. This also helps out the driver behind me: my rig maintains a constant speed, allowing others to more easily manage their speed and distance from my rear bumper.
In a Jeep, I would think that it would be a great substitute for a remote throttle control, the type clamped to the gear shifter. Select gear 3 or 4 low, rev to 1200 rpm, hit the cruise and let the computer easily get you up the hill without spinning, while you concentrate on your tack.