I think that lying in shabasana (corpse pose, not sure of the spelling) it would be possible to attain Jhana. It's basically just lying on the floor following the breath, but if the poster has trouble staying awake in the sitting posture he'd likely have even more trouble lying down! Anyway, Nicholas, this may be too obvious, but another thought occurred to me: if you're following the breath, when things start to get pleasant do you start to focus on the pleasant feeling right away? It's been my experience that if I switch to the pleasant sensation too early then the mind will begin to wander and I'll completely lose the object and end up in a dull state, but if I keep going back to the breath even when the pleasant feeling is trying to pull me away from it it will increase the pleasure and interest in the meditation. Anyway, you may very well know this and be way beyond my level, but I just thought I'd throw it out there.
"The heart of the path is quite easy. There’s no need to explain anything at length. Let go of love and hate and let things be. That’s all that I do in my own practice." - Ajahn Chah