I don't see why trying it with a small part of the ground would be such an issue. As long as it was managed correctly.
I hate standing for long periods of time (even at gigs) so i'm quite happy to bob up and down during 90 minutes but i do understand that fans should have the choice.
Stewards would have to make sure people who couldn't get into a standing area didn't just stand constantly in a seated area for 90 minutes as that would defeat the object.
When you watch the Premiership at home on Sky, do you all stand in front of your sofa's for 90 minutes?