I find Paul Chek's article, The Inner Unit, to be quite insightful. He recommends the four point maneuver and other stabilization exercises that are done in an "all fours" position. How does one modify the four point, for instance, for clients who are strong generally but whose wrists get sore before they complete their second set? Can they put their hands around a dumbbell and point their knuckles to the ground to keep their wrists neutral, or will that throw off postural positioning too much? Should we focus on forearm exercises first? What is recommended?
If someone's wrists become tired or sore, place a small medicine ball under each hand or, as you've suggested, use a dumbbell. The four point TVA activation exercises should only be done until clients can do them standing. The best position to try from standing is the bent-over row position while they rest their hands on their knees. This helps them relax the abdominals, allowing a better stretch of the abdominal wall by the visceral as they inhale, which facilitates the contraction. As soon as possible, you can implement the principles inner unit/outer unit integration described in my Scientific Core Conditioning course.