L-sit/V-sit GTG?

Does the grease-the-groove method work for L-sit/V-sit?

It very well could.

Yep. It works quite well. Last year, I would grease my 10 second l-sit throughout the day and it progressed nicely as the months went by. Eventually, I was able to go into a 7 second v-sit having never practiced one - only greasing the l-sit with hips forward.

Make sure you warm up your body first, though. Sometimes It would slip my mind and I would hear the scariest noises coming out of my elbows.