You definitely can borrow when making a reservation online. The online system tells you which points are available for borrowing. And you can even select the contract you want to borrow the points from by moving it to the top of the list of your contracts.
Just keep in mind once borrowed the points stay borrowed and expire with the new UY.
Thanks so much for your replies. I am looking to book Easter week of next year, and am happy to know that you can book (and borrow) online. That extra hour is going to make a difference for that very popular week!