I decided to go with stainless steel worm clamps around the plastic that encloses the brass inserts (see photo). This fix was cheap ($2 per clamp) and easy to do. The clamp should prevent further splitting and separation of the plastic.
Even so, overtightening the screws could still cause the brass insert to pop out. Midnight Dan is quite correct that very little force is needed to hold the leg extension securely (about 1/4 turn after feeling initial resistance).