I know this is an old thread, but just to add a data point.

I did the above where I reserved award seats until I got a PNR and then called in to the Premium Services Line. The rep and supervisor were not able to split payments. They were quite nice and courteous, I fully believe they would have if they could have (or knew the trick to do it). They did suggest splitting up the reservation into separates ones to accomplish something like a split payment. This thread also mentions that it may take up to ten phone calls to find the one person that knows how to split payments. I only tried one round

Additional info- some people (myself included) are trying to burn up credit card air credits. So using PayPal and other third party wallets can be problematic for that.