I did it a month or two ago.
The key is to not ask if you can do it.
The cashiers will say no, but the system allows you to do it.
I had a $50 one I took a chance with.
Got the WDW GCard and handed it to the cashier to scan.
Kept my WGreens GCard concealed in my hand until time to swipe.
Swiped it with my hand covering the face of it, and the register applied the payment with no problem.
Now, with you having that many points, I don't know if I would try too many at a time, just because it may draw unwanted attention to the fact that you are using them.
Then you may have the cashier say you can't do it, when in fact, you can.
I, personally, just would not want to deal with that aggravation.