Well, what if you sprinted at 90-95% at the end of an LT and then did some strides like 4-8 x 100 @ 400-800 pace?
I've found that learning how to sprint when you're tired can be more important than learning how to sprint. Two years ago I would always run a 400 all out at the end of my V02 max workouts. It carried over into races and I would always have a really strong, long, kick. Last year I decided to work on my pure sprint speed and form by doing workouts like 16 x 100 @ 200-400 pace using good form. My max speed and my form improved, but I didn't have that long kicking ability that I had had the year before. I now think it is important to practice kicking hard at the end of workouts while also doing workouts to improve max speed. I am just wondering if threshold workouts would be good workouts to practice kicking in without defeating their purpose.