http://194.80.240.74/images/events/R...20Dreaming.pdf

Allan Hobson theorises that lucid dreaming is an intermediary state between REM and waking in terms of neuromodulation.

You're completely correct in that there is reciprocal inhibition between cholinergic and serotinergic modulatory centres, but it's possible that there is some sort of mid-ground in lucid dreaming. The main reasoning behind this is that other aspects of brain state are at an intermediary level in lucid dreaming, so it is likely that neuromodulation is also in some sort of mid-ground.