Is it possible or even effective to practice Hung-Sau
Technique in a reclined position (granted, of course, that one is well rested enough not to fall asleep)?
Is it possible to meditate in a reclining position? Yes and no. Yes, in the sense that something very relaxing and soothing can take place while you’re reclining, even if you don’t fall asleep. But no in the sense that true meditation is far beyond mere relaxation: It is a dynamic state of awareness. Sitting upright with a straight spine is a "dynamic" position of the body that will help align you with that dynamic state, whereas the reclining position is a relatively passive position that tends to take you in the opposite direction, even if you can stay awake
Certainly some people have no other option than to recline, due to back injuries or other conditions. That does not preclude them meditating, but it will take a deeper, more determined effort for them to meditate deeply. So if you don’t have to recline, and if you want the phrase "as effective as it can possibly be" to describe your meditation, then learn to sit upright with a straight spine. Then you won’t be fighting against the natural tendencies (not only sleep, but general dissipation of energy) that come with reclining. You might need to practice yoga postures or other back-strengthening techniques to enable you to do with without strain, but it’s well worth the time and effort.
Blessings on your practice,