This should reveal whether or not Cruz is still an outsider or whether he has been co-opted by the GOPe
Yup, my bet is that he's compromised.
If Cruz doesn't back this, it means that he's not even serious about winning the nomination, but rather he's running just to give the RNC the brokered convention it needs to get one of its own in.
However, as taxcontrol said, supporting this rule would be purely symbolic, the candidates don't get any say, the delegates do. And if the delegates are mostly RNC hacks, they'll change the rule no matter what Cruz says.
And if DT does not, what does that mean? He didn’t get the news from Time?