Is 'learning to fall' something that climbers do through practice or
through on-the-job training, as it were?
A little of both. You can go to your local gym or outside on routes that you feel comfortable climbing and once you get a few feet above a solid bolt or bomber piece of gear, let go. Read this article on practicing falling, and this one. Since you know you're going to fall, the fall won't feel the same as a surprise lead fall, but you can learn how to relax and react. You can continue doing this on various types of climbs (slab, crack, overhang, etc.) to get the hang of falling. After some practice, try pushing your limits (safely) on a project to get over the psychological aspect of falling. If falling is all you are thinking about when you're climbing, you won't enjoy the climb and you are much more likely to actually take a fall.
This isn't something I've seen addressed much. Is there a 'good fall'
technique for lead climbing? Are there things during the fall I should
The biggest things are relaxing, knowing where your rope should be in relation to your feet based on the last piece of gear, not fighting it once it's happened, don't grab the rope, don't grab gear, and being aware of your surroundings (i.e., potential obstacles you can hit if you should take a fall). Depending on what kind of climbing you're doing (slab, crack, overhanging, etc.), there are specific things you should be aware of.
First, a fall on low-angled slab is usually unpleasant even if you do everything right. However, in the event you do:
If it is steep enough, pat your way down with both your hands and feet. It's all about keeping everything off the wall as much as possible and not catching on anything on the way down.
If it's lower angled, skip down with your feet (do not jump, just light touches). Try to keep your balance and keep your feet moving and keep your knees bent so they ease the impact along the way and when you finally come to a stop.
Regardless of the type of slab, avoid any prolonged contact with the rock, try to avoid getting caught on anything, and if you ever get flipped as you're falling, tuck and roll.
Avoid any jams that might not come out if you fall so you don't break something or get flipped. If it's a wide crack (off-width), you may need to apply the same types of techniques for falling as you would climbing slab.
Avoid any jams or positions that will keep you from taking a clean fall. Watch your footwork and hands so you don't get in a position that will cause you to flip or get caught in the rope. If you're falling into a wall, relax the knees and elbows to soften the impact.
How does one deliberately set about learning how to cope with big
falls - higher fall factor than top-roping generally allows?
Practice and experience. Find some safe projects to push your limits on. Climb routes you are comfortable on, but place one less piece of gear. The biggest thing is learning to trust yourself and your gear. As you lead more, you'll start to develop the right mindset for it.