I salvaged a good-sized crepe from a street median work area but it has very little root ball. Dug standard hole for crepe this size with normal ball, worked good mulch and soil, staked crepe, soaked and used stimulator; wondering what else, if anything, can improve odds of it surviving.

You may have planted it too deep. You MUST be able to see the flare of the roots at the bottom of the trunk above the ground or the plant will rot at the base and die. If you cannot see the root flare above the ground, then before it gets any later I would redo the planting. Also if you have already dug a deep hole, you will need to compact the soil directly under the plant so it does not sink back down as the soil settles.

Understood. Bigger question for me is, will the major lack of roots, period, doom this tree, or is it inherently tough enough to rejuvenate itself from the damage done by being ripped out by a backhoe?

I see. You have nothing to lose but you want to save the plant. I would spray it with milk every 2 weeks. Even though it looks dead, the milk is a good "food" for the microbes that live on the outside of the plant. Spray every surface that is exposed, leaves, bark, everything.

You might have to stake it up loosely so it can wiggle if it needs to. Trees that are staked tightly do not develop roots that will hold it up. Loose means that you don't want it to blow down in a gale but it should be allowed to bend some in the wind. If you use guy wires, they should be very loose. I would lean toward a loose cord rather than wire. If you stake it up let it have some wiggle room.

Actually, this entire reply, not just the quote, is exactly what I needed. My thanks for a full answer. Fortunately, caught the tree soon after it had been torn out, so branches are still pliable. Will loosen guy lines [threaded thru hose sections] to allow waggle room and will gladly experiment with the milk suggestion. Whodathunkit?

Crape Myrtles are tough plants. At my kids' elementary school, the facilities crew ripped out about 5 very mature crape myrtles, about 20 year old trees, barely a rootball, and they transplanted them to line the fence of the soccer field, which has no sprinkler system. They did this in the late spring, which was the worst time to do it. I thought for sure they would die over the summer. This happened about 3 years ago. Well, except for getting some water from a compassionate neighbor of the school who would take buckets or sometimes drag a very long hose out there, those crape myrtles received no attention. They are currently thriving. I don't think you have too much to worry about. Any good soil drench will just make things better!