I don't see why this is true. The only thing that is different between a standup and standard sitdown coaster is the riding position, which wouldn't change g-forces, apples to apples. Because of this standups have large, more drawn-out transitions so that you're not putting massive loads of positive g's on someone's legs, so I don't see why you couldn't do something like a cobra roll as long as you could stretch out the transitions and had enough speed to do it. Which is exactly why I think we never saw it, you'd probably have to build one really, really big cobra roll to get a standup train safely through it, and why bother why you can do so much else instead?
Also if by heartline roll you mean something like the B&M camelback thing on their sitdown coasters, Drayton Manor's already does it -
http://rcdb.com/814.htm?p=8777
As you can see they just made it stretchier to make the transitions work.