"There comes a point where the extra velocity you could gain by using the escape rocket's thrust is offset by the energy required to carry the extra mass into orbit. Every American system that used launch escape rockets simply ejected them.

It's possible they could've fired them and used their thrust to help pull the stack but then you'd have to carefully choose when to blow the explosive bolts so the LES would fly clear before it ran out of thrust. Those LES rocket engines only fired for a few seconds so I don't know if the added thrust would've justified the extra risk. While not positive, I suspect the Russians also jettison their LES stacks."

Hmm only a few seconds, huh?
Well if they could be designed to fire for about 20 seconds, then perhaps they could always be lit just after main ignition. This way they could add to the thrust at blastoff and be used as emergency escape. This means that after about the 18th second they would be jettisoned.