''So why aren't we seeing CRJ1000's with 4-5000nm range? Surely it's just as simple as chucking 20 seats out above the wing, and sticking a fuel tank there, no? I see huge market opportunities if such an aircraft existed.''
Sure they exist. I've just flown one from Eastern Europe to New York. Global XRS. Actually the same tube as the CRJ, a lot more spacious because it doesn't have the overhead bins, mind.