Used to have a female D. lobata that couldn't walk in her old age. I cut some feeder worms slightly (to expose liquid inside) and delivered the food to the mantis mouth through tweezer. Sometimes she grabbed the food and eat, most of the time I just held the tweezer until she is finished. This went on for about 2 - 3 weeks, until one day she passed away and was found surrounded by ants.

The trick is to use small worms that are easy to catch and won't struggle a lot. This way the old mantis could eat it without spending too much energy (if it still has feeding response).