I suspect that the main issue would be the card fee. The bank might be OK with taking $50,000 on a credit card, but they won't take the hit for the $1000 or $1500 fee Amex would charge to process the transaction.
I have never actually done this, but one thing I've kept in my bag of tricks, is that I get a bonus for charging $30K on my gold PR card. If I hit 12/31 and I'm short by, say, $2000, I would charge $2000 of my taxes on my card. You pay a 2.49% "convenience fee" but if it puts me over the top for a 15K bonus, I would in effect be paying $50 for 17,000 points.