!!!!----I HAVE THE ANSWER----!!!!
I have searched for many years and tried so many recipes without getting anywhere near that magical flavour Finally I know why I could never succeed - it's done using charcoal burning in a dish floating in the saucepan of dal. To my surprise there are a lot of videos and photos and instructions out there and I cannot fathom why I never came across anything before except lots of other foodies on the same quest.
My favourite dal recipe so far is this one: http://www.theguardian.com/lifeandstyle/2011/jun/02/how-to-cook-perfect-dal, though it doesn't mention charcoal once.
I might try liquid smoke first and then a chunk of charcoal from a disposable barbecue. Then I can compare the results.
Anyway, here is the answer we've all been hoping for...
https://www.youtube.com/watch?v=KVzT1AQcGqk