I guess it depends on your definition of feeding on demand. I feed my baby when he's hungry which these days is approximately every 4 hours. I do not feed him according to the clock but I know to look for his hunger cues anywhere between 3.5-4.5 hours and will feed him accordingly - as I said, average he goes between feeds is every 4 hours. Before he started to feed every 4 hours, I found myself feeding very regularly and he was snacking and would never take a good feed as he still had milk in his tummy from an earlier feed. Although I have followed his cues, I have tried to gradually extend him to 4 hrs between feeds as that way he takes a good feed and will last longer in the day and night too.
I guess if you are co-sleeping then feeding on demand is probably easier as you can just roll over them and feed without really waking (I don't co-sleep) but my thinking on feeding on demand if they are snacking all day long would probably lead to many more night wakings as they never have a full tummy.
Whatever works for you really I guess but yes, I think you can make it work with EASY if you follow a rough pattern.