I would say that if you're set on mastering this ability, buy a set of $100 rollers and work your way up. At first, it'll be difficult to stay on them, then you'll improve the smoothness of your pedal stroke. Next, you'll be able to remove a water bottle, and some hours later, put it back. The next skill would be standing and pedaling... or riding rollers at low speed. You'll be amazed at how, in just a few weeks of exclusive roller use, your outdoor stability in all situations has dramatically improved.