My 2 cents:
Get some parallettes, push up handles or yoga blocks
Then sit cross legged (knees bent and ankles crossed) with your hands on the supports right by your hips.
Start by pressing down with your hands and your heels and straightening your arms as you lift just your butt off the floor.
Do a few reps of these to build your arms (like cheating dips). Then as your arms are straight and butt is lifted. Curl your knees into your chest to lift the feet off the floor. (You're half way there!) Your back should be straight up and down (perpendicular to the floor and right between your arms and the knees should be drawing up closer to your chest. Hold these for as long as you can. Maybe this is your L-Sit substitution.
Eventually to work the legs straight... I would lift up with knees bent keep your ankles crossed and begin to squeeze your legs straight. As you squeeze the knees and inner thighs together the feet shoot out in front of you. Eventually work to uncrossing the ankles and keeping the legs together just using your adductors.
You can also play with keeping the hips lifted and slowly drawing the knees into the chest and then straightening the legs, then bending, then straightening. One breath in, one breath out.
Hope this helps.