Some airlines already utilise 737NGs or A319LR in their schedules for long-range flights. (KLM/LH/LX all use Privatair aircraft I believe).
However, these aircraft are fitted with all business class seats to make the costs work. 150 economy seats wouldn't bring in the revenue and the aircraft would possibly have performance restrictions as a result.
At the moment, these aircraft generally fly between major airports as that is where the demand is. I don't believe that a carrier could make money on other routes... or they'd be doing it already. Just because a few people would find such a route a bit easier for their work, doesn't mean that an airline could consistently fill seats in both directions a few times a week.