When I tried to spar recently I found it really really hard to stop the shoot for my legs. MMA people are good at this - ie they change the level. Wing Chun doesn't.
I thought maybe this idea could be worked either on the bottom arm or the leg even ! You simulate pummeling for under-hooks [I need to keep elbows in here] and then drop your hips a level and shift your legs back. You're meant to simulate the force driving you back as you prevent your hips being forced back and your legs grabbed.
I was a bit half-hearted in this as I didn't want my hands to slip and break my face but with practice I bet you could get quite aggressive on this and even drop to the dummy leg level. It is just an idea. ( I wanted to give it a try to drop to leg level but the camera would need readjusting and it was getting dark).